Usually it's permission or dependency problem. If you're not sure if you have all dependency, install it with command
sudo apt-get install python3-pyqt5
If you build from source, install the dependency and install Electrum parameter
--upgradesudo apt-get install python3-setuptools python3-pip
python3 -m pip install --user Electrum-3.3.6.tar.gz[fast] --upgrade
If you still can't run electrum, try to get error message by running electrum from terminal so we can diagnose the problem.