In ubuntu 20.04, how to fix pip path?

On Ubuntu 20.04 simply log out of your account and log back in to fix that warning. This will re-evaluate your ~/.profile, which in turn automatically adds the ~/.local/bin folder to your path if it exists (which it does now, but probably hasn't before your first run of pip3 install).

After logging back in, virtualenv should work as expected and later runs of pip3 should not raise that warning anymore.

My answer fixed the issue but shouldn't have been the right answer. If you happened to be here because you are currently facing the very same issue that OP met, please refer to Dreamer answer first (and comments)

virtualenv is not in your PATH for some reason (not properly installed ? I believe it should be in your /user/local/bin/ or /user/bin/, I see you didn't use sudo ?). You can add it to your PATH with

echo "export PATH=\"/home/mark/.local/bin:\$PATH\"" >> ~/.bashrc && source ~/.bashrc

Edit : pip3 install PKG --no-warn-script-location to use the flag to ignore the warning