How to fix: Tired of seeing this? Rename your personal Microsoft account
You may get this error message if you have two Microsoft accounts under the same email address. A few years ago Microsoft made an attempt at unifying their account types but didn't quite figure it out and seem to think this error message to end users was a solution. Luckily it's not too hard to solve.
You can get this error in Speaking Email when logging in using "Office 365 Direct". You have two options to solve this:
1. Change to "Office 365 ActiveSync Modern Auth" if you want to use your work account only. This login method doesn't support personal Microsoft account, so you won't get this problem. However, this uses ActiveSync, which is an older technology and not quite as good. (It's slightly slower and slightly less robust, sometimes needing a resync.)
2. Follow the Microsoft recommendation and change the email address of your personal account to an alias so the Microsoft security system doesn't get confused and keep popping up prompts. This is simple to do and will prevent this prompt popping up on any application that uses your Microsoft account.
Click here to follow the Microsoft instructions to create an alias and make it your primary account.
The screenshot below shows doing this: