Why is it saying my email address is not valid for an Apple ID

The only thing I can imagine, as this article says:

Change your Apple ID - Apple Support

is:

You can change your Apple ID to any other email address you control, as long as it's not already in use as an Apple ID. If your email address ends with @icloud.com, @me.com, or @mac.com, you might not be able to change your Apple ID. You also won't see Change Email Address on your account page.

If that doesn't explain it, then I suggest contacting Apple Support using the link at the bottom of the above link.

Typically that means it is in use for another Apple ID of yours.

A rescue email or a reachable-at email.