Invalid email

It sounds like you are trying to change your Apple ID to one that cannot be used. This situation often develops because the Apple ID is being used by someone else or you are trying to use an Apple ID that is, or was, in some way, associated with another Apple ID of yours (e.g., as a reachable at email address or a notifications email address) in recent history.

Note that you can't reuse an email address as described in the second case until an uncertain period has passed--sometimes reputed to 30 day and sometimes 1 year. Also, I have seen that you never can use such an email address again. The last two times I saw someone claim a time period it was “up to 30 days.” I believe that up to 30 days is the current answer. But, you can contact Apple support to check out the currently quoted wait period.

 Get Apple telephone support for help using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 Be patient sometimes Apple support is closed to immediate phone support.

Some email addresses that were removed from an Apple ID can be reused after a year has passed.

Did you just abandon the Apple ID or did you officially delete the ID?

ASC - Late 2012 Mac mini, macOS 10.14.6; Watch, series 5 GPS/cellular, watchOS 6.2; iPad Air 2, ipadOS 13.5.1; iPhone 8+, iOS 13.5.1; TV 4th Gen, tvOS 13.4.6;  HomePods, audioOS 13.4.6 and Airport Express w/ Airplay 2

There have been posts by people who have contacted Apple Support and they were told that if you delete an ID associated with an email address, you can never reuse the email address for an Apple ID.  I have also seen posts that stated Apple told them there was a 30 day waiting period before the address could be used again.You will need to get another e-mail address to use as your Apple ID or you can try waiting 30 days to see if that works.