Most likely, that email is linked to another account already.
Open an incognito window where you're logged out and try to LOGIN with that email address. If it succeeds, that email is linked to another account already. You will then need to unlink your email from that account to link it.
Most likely, that email is linked to another account already.
Open an incognito window where you're logged out and try to LOGIN with that email address. If it succeeds, that email is linked to another account already. You will then need to unlink your email from that account to link it.