Re: Sims can't use modular stairs
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.