Weight Definition - Assembly Design

Hello Everyone. Im trying to define the weigth on this Part, but I don't unterstand why the Part doesnt accept the Material definition (see picture).
I had a Face Modell in the geometrical Set and then I create the Partbody but CATIA doesnt recognize that it is a Volumen Part.
do you have an Idea what kann I do to solve this Problem?
Kind regards an thank you for the help!