can't add gmail to outlook when using a security key

Outlook's sign-in window is a cut-down embedded browser and a lot of versions just don't do security keys. Google only offers the methods it thinks that window can handle, so you end up with nothing that works. Easiest fix is adding a second 2-step method on the Google account, the Google prompt on your phone or an authenticator app. Then when Outlook asks, hit Try another way and use that. The key keeps working everywhere else.