It's not called dos, not even DOS or MS-DOS, it's called command prompt.

As I answered 2 weeks ago in your earlier post:
"In a command prompt window I can run cmd. It's c:\windows\system32\cmd.exe and that folder is in my path. So it's not clear why the mvn -version gives this error message."

What is your path set to? And is cmd.exe in one of those folders?