String command;           //Define the string variable "command"

void setup() {
  // put your setup code here, to run once:

  pinMode(5, OUTPUT);     //Set pin 5 as MOSFET Gate signal pin
  pinMode(13, OUTPUT);    //Set pin 13 to use onboard LED as a waiting for command status LED
  digitalWrite(5, LOW);   //Start with MOSFET off
  digitalWrite(13, LOW);  //Star with status LED off
  Serial.begin(9600);     //Start Serial comm for sending commands

}

void loop() {
  // put your main code here, to run repeatedly:

  Serial.println("What's your command?"); //Ask for a command in the Serial Monitor

  while (Serial.available()==0) {        //Wait for user input stopping the code
    digitalWrite(13, HIGH);               //Blink the onboard LED indicating that the code is waiting for user input
    delay(100);
    digitalWrite(13, LOW);
    delay(100);
  }

  command=Serial.readString();          //Save the Serial Monitor String command to the "command" variable as a String
  Serial.println("data read");          //Confirm Serial Monitor input
  Serial.println(command);              //Print the command sent over the Serial Monitor to confirm the data received
  
  if (command=="On") {                  //Turn the MOSFET on
    digitalWrite(5, HIGH);
  }
   if (command=="Off") {                  //Turn the MOSFET off
    digitalWrite(5, LOW);
  }
  else {                                 //Provide feedback for a wrong command
    Serial.println("Wrong command!");
  }
}

int command;           //Define the int variable "command"

void setup() {
  // put your setup code here, to run once:

  pinMode(5, OUTPUT);     //Set pin 5 as MOSFET Gate signal pin
  pinMode(13, OUTPUT);    //Set pin 13 to use onboard LED as a waiting for command status LED
  digitalWrite(5, LOW);   //Start with MOSFET off
  digitalWrite(13, LOW);  //Star with status LED off
  Serial.begin(9600);     //Start Serial comm for sending commands

}

void loop() {
  // put your main code here, to run repeatedly:

  Serial.println("What's your command?"); //Ask for a command in the Serial Monitor

  while (Serial.available()==0) {        //Wait for user input stopping the code
    digitalWrite(13, HIGH);               //Blink the onboard LED indicating that the code is waiting for user input
    delay(100);
    digitalWrite(13, LOW);
    delay(100);
  }

  command=Serial.parseInt();          //Save the Serial Monitor imput command to the "command" variable as an int

  Serial.println("data read");          //Confirm Serial Monitor input
  Serial.println(command);              //Print the command sent over the Serial Monitor to confirm the data received
  
  if (command==1) {                  //Turn the MOSFET on
    digitalWrite(5, HIGH);
  } 
  if (command==2) {                  //Turn the MOSFET off
    digitalWrite(5, LOW);
  }
  else {                                 //Provide feedback for a wrong command
    Serial.println("Wrong command!");
  }
}

    String command;           //Define the string variable "command"

void setup() {
  // put your setup code here, to run once:

  pinMode(5, OUTPUT);     //Set pin 5 as MOSFET Gate signal pin
  pinMode(13, OUTPUT);    //Set pin 13 to use onboard LED as a waiting for command status LED
  digitalWrite(5, LOW);   //Start with MOSFET off
  digitalWrite(13, LOW);  //Star with status LED off
  Serial.begin(9600);     //Start Serial comm for sending commands

}

void loop() {
  // put your main code here, to run repeatedly:

  Serial.println("What's your command?"); //Ask for a command in the Serial Monitor

  while (Serial.available()==0) {        //Wait for user input stopping the code
    digitalWrite(13, HIGH);               //Blink the onboard LED indicating that the code is waiting for user input
    delay(100);
    digitalWrite(13, LOW);
    delay(100);
  }

  command=Serial.readString();          //Save the Serial Monitor String command to the "command" variable as a String
  Serial.println("data read");          //Confirm Serial Monitor input
  Serial.println(command);              //Print the command sent over the Serial Monitor to confirm the data received
  
  if (command=="On") {                  //Turn the MOSFET on
    digitalWrite(5, HIGH);
  } else if (command=="Off") {                  //Turn the MOSFET off
    digitalWrite(5, LOW);
  }
  else {                                 //Provide feedback for a wrong command
    Serial.println("Wrong command!");
  }
}

int command;           //Define the int variable "command"

void setup() {
  // put your setup code here, to run once:

  pinMode(5, OUTPUT);     //Set pin 5 as MOSFET Gate signal pin
  pinMode(13, OUTPUT);    //Set pin 13 to use onboard LED as a waiting for command status LED
  digitalWrite(5, LOW);   //Start with MOSFET off
  digitalWrite(13, LOW);  //Star with status LED off
  Serial.begin(9600);     //Start Serial comm for sending commands

}

void loop() {
  // put your main code here, to run repeatedly:

  Serial.println("What's your command?"); //Ask for a command in the Serial Monitor

  while (Serial.available()==0) {        //Wait for user input stopping the code
    digitalWrite(13, HIGH);               //Blink the onboard LED indicating that the code is waiting for user input
    delay(100);
    digitalWrite(13, LOW);
    delay(100);
  }

  command=Serial.parseInt();          //Save the Serial Monitor imput command to the "command" variable as an int

  Serial.println("data read");          //Confirm Serial Monitor input
  Serial.println(command);              //Print the command sent over the Serial Monitor to confirm the data received
  
  if (command==1) {                  //Turn the MOSFET on
    digitalWrite(5, HIGH);
  } 
  if (command==2) {                  //Turn the MOSFET off
    digitalWrite(5, LOW);
  }
  else {                                 //Provide feedback for a wrong command
    Serial.println("Wrong command!");
  }
}

And the Serial Monitor output:

What's your command?
data read
1
What's your command?
data read
0
Wrong command!
What's your command?
data read
2
What's your command?
data read
0
Wrong command!
What's your command?

Also, anyone knows how to avoid the multiple Serial Monitor outputs?

I tested this with "String" variables to be able to get this fixed.

EDIT: After trying to use the "else" statement as recommended by per1234, the issue continues to happen, but changing the variable type to an integer works, although the Serial output is rather messy:

int command;           //Define the int variable "command"

void setup() {
  // put your setup code here, to run once:

  pinMode(5, OUTPUT);     //Set pin 5 as MOSFET Gate signal pin
  pinMode(13, OUTPUT);    //Set pin 13 to use onboard LED as a waiting for command status LED
  digitalWrite(5, LOW);   //Start with MOSFET off
  digitalWrite(13, LOW);  //Star with status LED off
  Serial.begin(9600);     //Start Serial comm for sending commands

}

void loop() {
  // put your main code here, to run repeatedly:

  Serial.println("What's your command?"); //Ask for a command in the Serial Monitor

  while (Serial.available()==0) {        //Wait for user input stopping the code
    digitalWrite(13, HIGH);               //Blink the onboard LED indicating that the code is waiting for user input
    delay(100);
    digitalWrite(13, LOW);
    delay(100);
  }

  command=Serial.parseInt();          //Save the Serial Monitor imput command to the "command" variable as an int

  Serial.println("data read");          //Confirm Serial Monitor input
  Serial.println(command);              //Print the command sent over the Serial Monitor to confirm the data received
  
  if (command==1) {                  //Turn the MOSFET on
    digitalWrite(5, HIGH);
  } 
  if (command==2) {                  //Turn the MOSFET off
    digitalWrite(5, LOW);
  }
  else {                                 //Provide feedback for a wrong command
    Serial.println("Wrong command!");
  }
}

Does anyone experience this issue? I even tried rolling back my IDE to 1.8.3, which I used to write the code the first time, then after a library update, and also getting version 1.8.5, it stopped working.

Also, a side question regarding the post size, should I remove the original code when editing, or should I move it to another answer?

Thank you.