bagel$ pip3 install --user virtualenv
  <snipped>
  WARNING: The script virtualenv is installed in '/Users/bagel/Library/Python/3.7/bin' which is not on PATH.
  Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
Successfully installed virtualenv-20.0.1

bagel$ pip3 list
Package            Version
------------------ -------
appdirs            1.4.3  
filelock           3.0.12 
importlib-metadata 1.5.0  
pip                19.3.1 
setuptools         42.0.2 
six                1.14.0 
virtualenv         20.0.1 
wheel              0.33.6 
zipp               2.2.0  

bagel$ which python2.7
/usr/bin/python2.7

bagel$ ~/Library/Python/3.7/bin/virtualenv -p python2.7 venv
PermissionError: [Errno 1] Operation not permitted: '/Users/bagel/Documents/projects/test/venv/bin/python'