You’ve installed the 3rd-party library into a virtualenv, but PyCharm doesn’t know about that by default. If nothing is specified, it will choose the system Python install as the interpreter. You need to go into the project settings and configure the interpreter to point at the virtualenv. PyCharm will then index the interpreter and allow you to autocomplete.
The virtualenv may be auto-detected in the dropdown menu on the left. If it is not, click the gear to the right, click “Add local”, and select /path/to/virtualenv/bin/python
(or \Path\to\virtualenv\Scripts\python.exe
on Windows).