Because there is no directory named Desktop in your current directory. Linux have a little different philosophy, and you can't get to Desktop in this way from every place. You can get instead into home directory by typing cd ~ from every place and to root directory by typing cd / from every place. Every other time you have to cd to absolute path (which starts with /) or relative path (which starts with ./ or with no character).

Because there is no directory named Desktop in your current directory. Linux have a little different philosophy, and you can't get to Desktop in this way from every place. You can get instead into home directory by typing cd ~ from every place and to root directory by typing cd / from every place -> you can think about it in that way, that Linux uses home directory instead of Desktop. Every other time you have to cd to absolute path (which starts with /) or relative path (which starts with ./ or with no character).

Example:

1. You are in directory /home/ninja (you can get your current directory by typing pwd).
2. You type ls. Console logs all directories which you can get into now by typing cd. E.g console logged: Download Documents Music, so you can type now cd Download to move there.
3. Additionally no matter where you currently are, you can use cd / and cd ~

Try that. ;)