@basma_salman
All the solutions you marked may be valid BUT not for this case... a 1 step is not a stair … the tool has so many constraints and variables which one may not so easily control...the Stair tool has always been a not so easy to handle and not user friendly tool and honestly you should think about how much time you have spent on this 1 step!
It may or may not be doable, either way its not worth the time and effort when there is an easy practical way to model what you want..What you need to do is:
1. Either model it as part of the upper slab
2. Or model it as an element In Place or as one of the Generic Model families (line based maybe)
The above two option give you more flexibility in controlling how it look like shape wise, graphics wise in 2D views and finishes wise in 3D when aligning the material surface patterns