The content area of the part that you are adding is empty.
Explanation
A part diagram has two areas: a content area that is identified by a white background and a blue border surrounding it and a sandbox area that consists of everything outside the content area. Only the elements in the content area are considered to be in the part. Any elements in the sandbox area are ignored when reusing the part. Parts with an empty content area have nothing to add to the sketch therefore are forbidden to be added.
User response
Add content to the part. If you have already added content to the part, ensure that you have saved your changes.