If you don't know I am a huge fan of Ubiquiti and UNIFI. Since some time I've been running on Unifi 5.9.X alpha/beta versions and one problem it has is that it specifies you need MongoDB on version 3.4. Unfortunately with Ubuntu 18.04 that's no longer the case. So if you try to install package it will complain and tell you that your MongoDB version is too high. You can of course overwrite it by telling dpkg to ignore dependencies and so I did and I'm happy user of Unifi 5.9.x. The problem with this approach is that when you do that next time you try to update your system (not Unifi) you will get an error that Unifi can't upgrade and so any updates to your system are denied until you fix them.