ValerieR63 I downloaded the lot and tested it. Found the problem. It's that 1x1 tile room on the 2nd floor that closes the gap between the foot of the stairs and the wall. Found 3 ways to fix it.
When you look down from above, there is no floor in that little room. And if you send a sim into the house in first person mode, you will see that there is no ceiling there. So, one way to fix it, is to put a floor in it. another way is to select the little room and delete it. I don't know why, but the walls on the stair side of that room remain so nothing changes except when you look down that little room, you will see the roof. Basically, you are left with a void there. And the 3rd way I found to fix it is redraw that 1x1 tile room. That will also put a floor in. After using one of those methods, when you go look at that spot from below in first person, you will have a ceiling there where you didn't before.
One note on the roofs. The person who made that lot took the easy way to do the roofs by using one roof across that floor so the roofs will run through the rooms. They did it will all the roofs on the 2nd floor. I originally thought it was a roof causing the problem as I've tested a number of builds for people where the problem was the same as yours - route blocks, and the roofs were the cause. Pulling the roofs back so they don't intrude into the rooms and using multiple roof segments rather than 1 segment fixes it. Since you have that situation on the 2nd floor, I suggest you pull those roofs back and use segments at each end rather than have the roofs go through the rooms on that floor.
Hope this helps.
