Sims can't go up the stairs

Try this: https://www.ea.com/games/the-sims/the-sims-4/pc/gallery/31AF57B2B40A424F9C47C2099D3EF963

The issue is the top of the bottom staircase - the last step before the platform is too high for the sim to stand on without removing the ceiling above it.

Interestingly, if you just draw the fence around those two tiles and delete the ceiling, your sims will be able to go upstairs but not downstairs. I discovered this was because of the (invisible/cutout by stairs) section of "room" that was sticking out at the top of the stairs and would be visible if the staircase was moved. When I deleted the entire existing fence, however, and then redrew the fence cutting right across the top of the upper stairs and over the two tiles necessary to remove above the lower stairs, then to the wall, making the entire area above the stairs into one room that can then have its floor deleted, my sims were able to go both up and down.