Why can't I access 'virtualenv'?
Both which virtualenv and find /usr/ -iname "virtualenv" | grep bin should return/usr/bin/virtualenv. virtualenv is not installed in the right directory. It should be installed in /usr/bin/virtualenv instead of /usr/local/bin/virtualenv so reinstall it.
sudo apt install --reinstall python-virtualenv
For concise information about virtualenv basic usage visit Virtual Environments — The Hitchhiker's Guide to Python.