Hey guys,
I got a problem while sculpting my mesh. I applied a shell on a cylinder to make it hollow. The top of the cylinder is open now and when I try to close it via “close holes” nothing happens.
Do you guys know what the problem is? I really need your help 
Let me know if you need additional info.
close holes is for fixing non manifold geometry holes. Not for putting a top on a cup.
easiest solution is to make a new cylinder.
or
you could use the slice brush to cut off the top. auto polygroups and split polygroups. delete the unused section. close holes on the remaining one.
ZModeler Edge Actions>Close Hole should work with that.