The terminal and notification center do not open, what's the problem?

Question

Python

Installed python 3.6. I wanted 3.6 to be called when using "python3", not 3.5. Through update-alternatives changed. The terminal and the update center stopped opening (when I change back to 3.5 everything is fine). What could be the problem?
PS: In 3.6, when trying to import a non-existent module, it pops up:
ModuleNotFoundError: No module named 'apt_pkg'
The same thing happens when you try to run update-manager through the terminal
apt-get install python3-apt says that the module is already installed and there are no updates, but as I understand it it's only for 3.5