You MUST use the same original file that you Efiled originally for the amended return, its got embedded EF data in it, you cannot use a copy.

If you want to save a copy of the original (wrong) return, save a copy of it with a different name (be sure you uncheck the EF boxes on the fed and state on that copy or you'll end up with a message of duplicate SSN in the EFcenter) , keep the actual original file with the original file name to use as the amended return.