New Outlook won't let me add my outlook email account.

If you make a @outlook.com email your primary alias, it will fix the issue while not affecting functionality. You will still be able to sign in to Microsoft services using your @gmail.com email if it is attached to your account.

Most definitely seems to be a bug. I didn't try returning my other e-mail to being the primary, as everything works as is I'm reticent to change things back.

I did notice that other features seem to have trouble with a non-Microsoft primary login email as well, the OneNote Feed couldn't load properly either (incorrectly complaining about requiring a corporate or school account to work).

Thanks for reply

I'm not trying to make Outlook the primary account, just trying to add it in so that I can use it. If I run a trouble shooter it just gives me meaningless numbers.

Interesting, but that change to make outlook.com account the primary sould not be necessary... this looks like a Microsoft Bug , doesn't it?.

On the other hand, did you encounter any issues when returning the other acount back to primary?

There is an answer if you read the entire thread.