Cannot perform Check In with error about deleted or inaccessible key object

Cannot perform Check In with error about deleted or inaccessible key object 

Error:

​​The object with the following key was deleted or inaccessible, wt.epm.EPMDocument: xxxxxxx

The part set for check in has a relation to an assembly which is not checked out. This prevents the check in


Check out the related assembly that has a reference to the part