You mean that you want the base (bottom) of the Wall to be at the same level vertically as the top surface of the Slab?
Try to place it by using the Gravity feature or the Surface Snap feature.
Or, if it is placed above it, just drag it vertically downward by its base to the top surface of the Slab.
It would be a good idea for you to put your ArchiCAD version an computer specs in your signature as some of these features (namely, Surface Snap) were introduced in later ArchiCAD versions.
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