As per OOTB restrictions a CAD object that has not been uploaded cannot be associated to a Part. The CAD object must first be successfully uploaded, then it can be associated to the desired Part. The OOTB error message is misleading and does not provide guidance on the root cause of the action being prevented. The error message states: "Error: Invalid object type for edit association action." (See screenshot) It would be more helpful to have an error message stating that the object must first be uploaded or checked in.