Can't Install Gnome-Control-Center on Ubuntu
To install GNOME Control Center you have to specify its version manually by using command below:
sudo apt-get install gnome-control-center gnome-control-center-data=3.36.5-0ubuntu2
to avoid conflict with previously installed 3.38.3-1pop1~1625337172~20.04~bdcf8ad version.
To correctly remove all trails of PopOS!, you have to re-add their PPA and then purge it by
sudo add-apt-repository ppa:system76/pop
sudo apt-get update
sudo apt-get install ppa-purge
sudo ppa-purge ppa:system76/pop