After you file

I only had one dependent, my daughter. Her dad wants to claim her instead. So I am trying to remove her as my dependent so that he can e-file his taxes. My amended return keeps getting rejected, even though my daughter is no longer listed as my dependent. And her dad’s return is getting rejected because “someone else claimed that dependent already.” I’m at a loss and starting to think my amendment and his return both just need to be filed by mail at this point.