How do I set traps in the kitchen ceiling when my roof is flat and I lack access?
Although you cannot access the area between the ceiling and roof, you should certainly give all due effort to finding possible point(s) of entry for the mice. For one thing, you can bait or place traps nearby (trial and error here might also help to identify whether or not your hunches are right about points-of-entry).

More importantly, trapping the mice will have limited benefit if you can't cut off access to other mice.