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For more information on how adding an event works for your Outlook or Office 365 account, please see the following help doc on which Mircrosoft calendar option you should use when adding events to your Outlook.com/Office 365/Outlook desktop calendar.

Action:

When a user clicks either "Outlook.com" or "Office 365" in an "Add to Calendar" button, the below errors might be triggered:

Error:

"Need admin approval" to access your calendar.
Error: 404. The event contains errors. REST API is not yet supported for this mailbox.

If you encounter either of these errors, you should contact your Office 365/Azure administrator and send them this help doc to review.

The issue is directly tied to the user's Office 365 accounts/install. When you, as an organization, buy and set up your Office 365 software, you can turn things on and off. To interact with the Microsoft Events API, the "third-party app access" needs to be turned on. When that has been done, our AddEvent app at Microsoft can connect to the user's calendar (if the user has granted access). This setting can easily be turned on, but it requires that the user contact their local Azure/Office 365 administrator to do so.

Below are some articles from Microsoft that describe the issue and what needs to be done by the administrator.

Overview article here:
https://docs.microsoft.com/en-us/office365/enterprise/integrated-apps-and-azure-ads

And more detailed here:
https://docs.microsoft.com/en-us/microsoft-365/admin/misc/integrated-apps?redirectSourcePath=%252farticle%252f7e453a40-66df-44ab-92a1-96786cb7fb34&view=o365-worldwide#__toc379982114