First, verify that you have entered the correct EIN from prior tax records of the client or the CP-575 letter.  If it is correct, did you remove the EFILE information from the 1120 in your system or delete the 1120 that you previously tried to file? If no, you must do so then try to file. If yes, and it is correct, you will need to mark the correct return "Amended" in order to file it.