You cannot create a user interface part from a derived control. Save the selected control from where it was originally defined.
Explanation
The element/control you are trying to save as a part was not originally created in the current artifact but is a derived version of another control defined elsewhere. Such derived versions of elements cannot be saved as parts; you have to save its original version.
User response
Select the element. In the decoration bar above the widget, click the left arrow repeatedly until you find the original widget. Save this widget as a part from there.