venv is a package shipped directly with python 3. So you don’t need to pip install anything.
virtualenv instead is an independent library available at https://virtualenv.pypa.io/en/stable/ and can be install with pip.
They solve the same problem and work in a very similar manner.
If you use python3 I suggest to avoid an “extra” dependency and just stick with venv.
Your error is probably because you use Python2/pip2.