If none of the solutions mentioned above work, it can be due to environment variables pointing to two different locations.
Just use where virtualenv
to find the paths that are linked to virtualenv command.
Run the following in cmd.
D:\>where virtualenv
The output will look like:
C:\Program Files\Python37\Scripts\virtualenv.exe
C:\Users\username\AppData\Local\Programs\Python\Python36\Scripts\virtualenv.exe
You can see there is conflicting installation of python36 and python37. I just deleted python37 folder and the conflict was resolved.