The fact that you can't use "check geometry" is odd. Try to see if you can access the tool from the menu bar: Flow Simulation > Tools > Check Geometry
Something else you can try is simply creating a rectangular shell around your model and setting the internal walls to ambient boundary conditions. This will increase the size of your domain but it would save you the trouble of hunting down and sealing leaks.
The check Geometry options are working fine today, yesterday the whole software was messed up.
I guess you cant expect even the licensed version to work properly.
i think i may have seen this before when you are in leak tracker, those features aren't available
in addition to evaluating your geometry closely, you can use the leak tracker to find where there are holes. for new users, i always recommend starting with just the outside, no components on the inside and then add parts afterwards.
and always do these checks