Recently Outlook users who already added their accounts to Outlook profiles and those who are trying to sign in using the accounts weren't able to do so because of the error message popping in, stating: "You can't sign in here with a personal account. Use your work or school account instead."
Microsoft is aware of the bug and is currently working on the fix. According to the company, the problem is related to how Outlook is authenticating for diagnostics in some situations.
For users who don't want to wait for the solution, Microsoft released an official workaround for the issue, recommending turning off Support Diagnostics, which turns off the option to submit an In App ticket using Help and then selecting Contact Support.