I put in the email address. I’ve only ever used contacts, I’ve never used the address book, so why would it be in address book when I’ve never used it?   

Madison,

            It expects you to enter the e-mail address, e.g., blah@....  If you don't know what the other e-mail address was, either search your e-mail for a message from that person to snag it from or contact them to get it again.  You clearly still have the address you're trying to enter in Address Book, or you wouldn't be getting the error message you are, and if you want it in Contacts and not Address Book it's going to have to come out of Address Book first.

            I'm really confused as to why you'd have the same contact information in both Contacts and Address book.  It is typically in one, the other, but not both.  I can never remember whether it's Contacts under Outlook that is shared across all defined accounts or Address Book.
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Brian - Windows 10 Pro, 64-Bit, Version 2004, Build 19041  

The purpose of education is not to validate ignorance but to overcome it.
       ~ Lawrence Krauss