Cannot sign in to AppleID

Whenever I attempt to sign in with my AppleID, I use the same email address I've used for years ( *****@mac.com). It works fine for setting up a Mac or a device, and for purchases, etc. But when I attempt to log in to appleid.apple.com I use this address, the proper password (which it accepts), and I respond with the 6-digit security code it sends me. All good so far.

But then it comes to a page saying to confirm my email address (which it has already populated correctly). I accept, and it invariably responds with the message " This email address is not available. Choose a different address." I don't want to use another address to change my Apple ID.

Any thoughts?