You cannot create a user interface part from multiple controls. Group the elements into a container such as a panel and then save the container as a user interface part instead.
Explanation
A part's content area can only contain a single element/control but that element can contain other elements.
User response
If you want to create a single part out of the selected elements put them all in a container element such as a window, group box, or panel and then save that container element as a part. Alternatively, if you want to create multiple parts select the elements one at a time and save them individually as different parts.