Thank you for your answer,Benson.
When I copy and paste the bench into a new file, the cursor selects the surfaces and I can extrude them with Push/Pull (alt+P).
When I am working with the main file (that contains all the layers, classes, x-refs etc) the red rectangle that blocks the extrusion keeps appearing.
Bildschirmvideo aufnehmen 2021-11-17 um 09.19.36.mov