First you draw the walls underneath the staircase.
For the straight wall you edit the profile of the wall.

The curved wall is a bit trickier. Here you need the ceate a component by Model-in-place. Draw a Void Sweep Blend along the curve of the wall. And cut the wall with this void before finishing the component.

I also tweaked your staircase a bit, to resemble your picture.
I can't post the file itself, as I'm working in Revit 2016.


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