CATIA R21 requires .NET Framework 3.5 (includes 2.0 and 3.0). On modern Windows 11, this is disabled by default.
If CATIA crashes on startup or shows a blank screen, apply these "Updated" fixes: install+catia+v5+r21+on+windows+11+updated
Windows 11’s WDDM 3.0 drivers can cause CATIA V5 R21 to display a black screen or flicker. CATIA R21 requires