Are you trying to pick the Face of the slab?
"Select by face" is a setting under the Modify > Select .

It's also on the status bar on the far right, bottom. Most things in Revit are selected by an edge but if you are trying to select something like a floor, which is usually under a wall, it is quite problematic. So turn on select by face and it will select the wall face instead of having to TAB key select at the edge.
Jay Colcombe
Autodesk Certified Instructor
Revit Architecture & Structure Certified Professional
AutoCAD Certified Professional
B.Sc. Hons Civil & Structural Engineering
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