It's an issue with MyAccount for some customers. I'll get the TalkTalk team aware that it's happening to you.
Check your Community Profile Personal Information (Click here) is up to date and includes your name, current TalkTalk landline 'phone number and an alternate number (mobile recommended) and address with postcode (location). Add, then scroll down and save changes. The TalkTalk Community Team OCE will then be able to link to your TalkTalk service account.
Just to confirm, you are putting just the first part (localpart) of the desired email address into the top box? i.e. you don't enter the @talktak.net bit. And getting the 'Sorry' message although there are green ticks to the right of both boxes?
In my experience it is worth retrying as that worked for me on the retry and the new email address was listed straight away.
Well, the fix was applied last weekend and for many that fixed the issue. It seems however from yourself and a few others that some are still affected. I've raised this back to the team responsible for it and will let you know when I get an update for you.