you can’t go and install apt packages without updating your system first You can use apt without updating first. You’ll just be installing potentially outdated versions if the cached repository information is old. Though you may run into issues if you do partial upgrades (updating the cached repository, but not running an upgrade, then installing something new). Though this is less of an issue on Ubuntu since they try not to do big updates.

you also can’t use a GUI apt frontend as well as apt via the command line The error message should tell you that you can’t have multiple commands running at once. The error message is a bit too technical, citing the lock files.