Sims can't use modular stairs

@koshechka84 

From you pics, I am not sure what the issue really is.  I don't understand what I am supposed to see in the Main Floor and Without Floor pics.  What I do see is that there is no floor in the room with the green floor so did you delete it to show in the pics or is that the way the house is built and it is supposed to look that way?  If it is supposed to look that way, then at which level is the stair supposed to come out on?  Another thing is that when you get the humps around the stair like you do in the Without Floors pic, that is usually indicative of there not being enough room for the stair.  Since your pets can go up and down but the sims can't is also indicative of there not being enough room for the stair, but it is a lack of head room that causes that.  I had a case where my toddlers could go up and down but the teens and above couldn't (didn't have any children in the household at the time so Idk for them).  I ended up having to move a wall on the upper floor back 1 tile to fix it so you might need to do that.  I suggest you play around a little with the wall configuration around the stair to see if that fixes it.

But I do have a couple of other suggestions.  Try lowering the basement level by grabbing the big arrow in the middle of the basement and drag it down a few clicks to see if that helps.  The stair configuration will probably change when you do that and you'll have to make adjusts.  You could also try raising the basement level one click at the time until the outline goes red then back down 1 click.  Just grab the arrow in the basement and page up to the 1st floor to watch the outline.  Red means you went too far.  Green is ok.

Another thing you can try is changing the basement level wall height.  It looks like you are using the lowest height so try changing it to the middle height or the tallest height.  Again, the stair configuration may change by doing that. 

If you would like me or another helper here to test your lot to see if one of us can figure out what is going on, you can upload it to the Gallery and just post here that you did so and provide the lot name so we can find it and download.

Hope this helps.

@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.

@koshechka84So, in the most recent pic you attached of "Main Floor", the adult Sims can't use the stairs to basement?

Have encountered a similar problem with stairs to basement in one of my builds. Was working fine until it wasn't. As I recall, I either had to change floor plan or stair-config a bit to get it to work again.

@stjmk Well, yes. Sims just stand and shout. I've tried changing config of the stais. Still can't use any of the variations.

For now I'm playing without flooring on the upper level of the stairs.

TO BE CLEAR MY ISSUE IS SIMILAR, BUT my sims CAN use stairs, but when they are bent (configured) they get stuck at the bend and sometimes teleport to where they were going, or sometimes just get hung up and take extra long to navigate. At any rate, there are def. routing issues present with configured stairs since the 7.20.21 patch.