Why can't I install multiple packages at the same time? ("Unable to lock administration directory")

Solution 1:

There is no inherent reason you cannot do multiple jobs at the same time.

But some jobs are not meant to run at the same time. This is usually implemented by locking something.

In this case, you can imagine an update needing the current state of your machine, but it has to be sure nothing changed. If you are updating something (installing package X) and you run a different update that needs package X as well, it will ALSO try to install that package

Therefore, apt (or aptitude, or anything using the Ubuntu package system) should not run at the same time, and uses this lock file mechanism to check.