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One of the common causes of the log-in error is that your Microsoft account is not recognized as a valid account for the apps. You may need to check if your account is linked to the apps and has the right permissions. To do this, you can follow these steps
• Open any of the desktop apps, such as Word or Excel, and click File > Account.
• Under User Information, make sure that your Microsoft account email address is displayed. If not, click Sign out and then Sign in with the correct account.
• Under Product Information, make sure that your Microsoft 365 Family subscription is activated. If not, click Activate Product and follow the instructions.
• Restart the app and try to log in again.
For further reference, you can also find similar issues and solutions in the link provided: https://answers.microsoft.com/en-us/msoffice/forum/all/cannot-sign-in-to-office-365-desktop-apps/a5f1d27c-0a86-4214-83fc-ed87180d670c
If you have any further questions or need additional assistance, please feel free to ask. I'm here to help!
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Ajibola.