Hi CarllyJay,
Welcome to the Apple Support Communities!
I understand that you would like to edit your Apple ID to a different email address, but you are receiving an alert that this email address is already an Apple ID. I also understand that you checked your current Apple ID to make sure the email address you would like to use is not set up as an alternate address to your current email address.
To begin troubleshooting, I would suggest trying to reset the Apple ID password for the email account that you would like to change to. It is possible that you have two email addresses with two independent Apple IDs. If you are able to reset the password, try to log in using that account.
If you forgot your Apple ID password - Apple Support
Best regards