Stephan, while above comments are very valid, you do appear to have a badely messed up install and life will be a great deal happier for you if you fix the underlying problems rather than 'patching' what you have.
I'd like to understand how your Lazarus install is so messed up ? Maybe back when 16.04 was a regarded as a cool thing, the default, repo provided an install was like that ? Or have you manually installed it yourself and, perhaps made a few mistakes ? I really don't recommend you use the default, Ubuntu repo, install of Lazarus on even 18.04, it will get you an out of date version too.
As Marcov mentioned, 16.04 is already pretty old. A new install of, eg 18.04 would be heaps better, supported for longer and when you ask questions on this forum, people wont say "16.04 ? why ?". In my experience, 18.04 places no extra load on your hardware, if thats a concern, I suggest a light version, I prefer to use Ubuntu Mate even on quite powerful hardware.
So, just a suggestion, maybe install some flavour of 18.04 and then have a look at
http://wiki.freepascal.org/Installing_Lazarus_on_LinuxDavo