There must be a reason for that, such as — for instance — that you would be trying to delete the user account that you are currently using, or a user account that is currently still logged in.
Either way, just because you cannot do it from a GUI doesn’t mean that you cannot try doing it from the command line… 
man userdel
That’s an old problem, which you wouldn’t have run into if you had kept your system up to date. Please remember that Manjaro is a (curated) rolling-release distribution and must be kept up to date, and must be maintained — you probably also have loads of unattended .pacnew files on your system.
Please take the time to read this short essay below — it’s not meant to berate you, but rather to bring your attention onto what Manjaro requires.
Anyway, I don’t have any link handy with regard to the openjdk issue, but it was abundantly addressed on the forum already, as well as on the pertinent Announcements > Stable Updates thread when the change was introduced.
Yes, by way of userdel — see higher up.