@Sanaa Al-Tamimi, Thanks for posting in Q&A. For your issue, I would like to confirm if we have set conditional access policy in our organization. How did we set under Grant? Did we set to "require app protection policy"? If yes, please ensure app protection policy has been applied to this user's Outlook.
https://learn.microsoft.com/en-us/entra/identity/conditional-access/concept-conditional-access-grant
Meanwhile, from your description, I know your issue is urgent. For such case, you can consider opening case to work on it. Here is a link with the steps to open case for your reference:
https://learn.microsoft.com/en-us/mem/get-support
Please check the above information and if there's any update, feel free to let us know.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.