Error while using interface export in same typescript file

If I use interface and export class in same typescript file then I'm getting exception like follows.

Sample Code:

interface Window {
    Test: string;
}

window.Test = "cool";

export class Person {
    name: string;

    constructor(name: string) {
        this.name = name;
    }

    print(): void {
        console.log(this.name);
    }
}

Exception : Property 'Test' does not exist on type 'Window & typeof globalThis'.ts(2339)

if I remove class definition then error banishes. I googled for many hours but unable to find the reason. Could someone describe the reason behind this?

PS: I'm a newbie to Typescript and I'm exploring it to integrate it to my existing project. Pardon me, If my question is too amateurish.