Sims can't use modular stairs

@CGrant56 Due to slow internet it seems that wrong picture was finally attached. Of course there is flooring on the Main Floor. When I was trying to correct this I accidentally clicked to Remove Floor, and after that there was that unlevelled terrain near the stairs. So I captured this too.

Well, you see, It worked, and for a long time. I played this household with this partivular stairs. And there was no any issue with them. But somewhere between recent patches (when I switched to other HHs) it broke.

Thank you for trying to help)
In my test game I've tried lowering levels/ raising levels of the basment itself, of the walls, of the foundation. Well, every your suggestion) Nothing works. I've deleted the stairs to build it from the beginning, Everything works up to the moment of shaping the stairs into several bends. I've removed walls aroun the stairs, still nothing. BUT... When I just delete the single flooring tile in this small stair room, sims start normally using it. So I'm playing with the green grass instead of floor for the moment.

I'm not much of a builder, so to build this basement was really tough for me 😳

And as I understand, there was something with using the stairs, (qoute: We have fixed an issue where Sims could not use stairs when Simmers deleted the floor of a fence block that was built around the stairs in the upper landing.)

So maybe this is a result of that or similar problem. And maybe this is somehow connected with building a basement in a already existing house with foundation.

This is just my thoughts, though.