Can't contribute changing the pseudonym for the stuff member

The movie title "The Meateater" (1979) https://www.imdb.com/title/tt0079541/, the name for the director/writer/editor David Burton Morris (as a different name). Is mane sources the pseudonym is indicated as Derek Savage. But when I try to make the contribution where the attribute field is corrected (as Derek Savage), or (written by) (as Derek Savage) and (story) (as Derek Savage) - it cannot be accepted with the reason "The as attribute "as Derek Savage" is not eligible for submission." Can't undestand what am I doing wrong.