Edit #2 Solution found; see below.
I\'m writing a small application in Flask using VirtualEnv. This is far from the first time I\'ve done this, but
I had the same problem. For me, the cause was that my virtualenv had spaces in the path.
Moving the virtualenv to a spaceless path solved the problem.
Use this link (Python packages not installing in virtualenv using pip).
If your shebang is ok then ensure you do not use "sudo -H" when installing to the virtual environment.
If you have renamed your project directory containing ENV - virtual environment directory, try wiping out ENV directory and recreate virtualenv and activate it and reinstall pip dependencies.
TL-DR; Delete virtual environment, create new one, activate it and issue pip commands again.