Invalid Python SDK Error while using python 3.4 on PyCharm

I had the same issue. Try to comment/remove the PYTHONPATH variable in your ~/.bash_profile

#export PYTHONPATH=/usr/local/lib/python2.7/site-packages/

If it does not help it also may be useful to look in the idea.log for the errors:

/Users/username/Library/Logs/PyCharm40/idea.log

I had the following errors:

Your PYTHONPATH points to a site-packages dir for Python 2.x but you are running Python 3.x!
     PYTHONPATH is currently: "/usr/local/lib/python2.7/site-packages/"
     You should `unset PYTHONPATH` to fix this.

This also happened to me. I renamed a repository and then my virtual environment got stuck in the old path.

I grepped all configuration files and could not find any reference to the old one.

What finally solved the problem was to clear caches with the option File > Invalidate Caches / Restart...: