Parapet

If your building already has a roof, this won't show you much. Draw a box (or a few) with nothing on top first, then come back here.

Set those boxes to Room for a more realistic result — select them, then change Building Role under General in the right panel.

Press Polygon (or type polygon) and draw along the boundary you want it to follow.

Misplaced a point? Ctrl+Z takes just that point back — you don't have to start over.

With it selected, open Details in the right panel → + Add Generator → Parapet Generator, then tune Parapet Height and Edges.