iPHONE 7Plus trading in on Iphone 13, Apple states "This device currently has Find My is turned on. HOw do I get it turned off if 7 Plus has been erased?

Has anyone been told to erase the old phone to return to Apple for a trade in on a new one and after erasing it and trying to return it are told by Apple that the Find My Phone was left on? How could that be if the phone is erased? Apple nor the person that sold me the new phone told me to make sure the old phone needed to be unlocked only to erase the phone and I did just that. Apple will not take my phone back now? Any suggestions? So frustraded and discouraged. thanks!!