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

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.

Yes i have the same problem with my uk kcom karoo account. old outlook work fine with my provider.

Try this, it fixed it for me: https://answers.microsoft.com/en-us/outlook_com/forum/outlk_win-outtop_classic-outsub_ofh/new-outlook-wont-let-me-add-my-outlook-email/11b1382f-87b4-485d-99c6-e316ab1de8e8?messageId=6b909355-e25f-4d7f-8a1c-31d9944f4782&page=1#:~:text=SOLVED!%20(hopefully)%20What,works%20for%20you. (If that link doesn't work then just look at my post on October 4th on page 1)

My Karoo account does not even show up after doing this. I use karoo through imap not pop3 will look at a clean installation of mail using the pop 3