Sometimes a desktop entry (launcher) for PyCharm gets broken as a result of upgrading your Ubuntu Linux system, or updating PyCharm.
1. remove all copies of the desktop entry from the system
sudo find / -name 'jetbrains-pycharm.desktop' -delete
(or skip the -delete option if you want to rm the files manually)
2a. if you lost track of where to manually open PyCharm,
sudo updatedb && locate pycharm.sh
2b. open PyCharm manually
and use Tools > Create Desktop Entry to create a new, working desktop entry. The "for all users" option is not necessary unless you really have multiple pycharm users on your system.
3. pin the new desktop entry to the launcher.