It seems to be a
$PATH related issue(message: command not found).
pip must be installing things on
~/bin, the bin directories inside your home.
You will have to add that location by editing the
.profile file inside your home, and adding the following line at the end of that file:
source ~/.profile should be enough to reload your profile file.
Take a look at the Bug #1588562 link below. It should be fixed in Ubuntu 18.04 but there is a guy reporting that it's not.