@cookies ^_^, Thanks for posting in Q&A. From the message, it seems the device is managed by Intune.
To remove this, please check if there's company portal or management profile on the device. If yes, unenroll the device in company portal and remove the management profile.
However, if the device is not enrolled in Intune, but the issue still persists, it can be that there are app protection policy or app configuration policy applied to work account. So when you access outlook via the work account before, the policy was applied to outlook which block personal account access. You can remove outlook, and reinstall it again, then only sign in personal account to see if it can work..
Please try the above suggestion and if there's any update, feel free to let us know.
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