Issue:
Attempts to add a specific shared mailbox hosted on Microsoft 365 Exchange result in the error: "Cannot open this mailbox | Unable to add account. Please try using the Outlook Web App."
Action is performed in the following path: Tools (menu) / Accounts / 365 account / Delegation and Sharing / Shared With Me / + / Search (text entry) / Add (button)
This issue may only affect some shared mailboxes, while others can be added without error.
(Example image)
Opting for the Microsoft admin interface: https://admin.microsoft.com/
Expand "Teams & Groups"
Select "Shared mailboxes"
In the "Search this list" box, enter the email address or name of the affected shared mailbox.
Click the name of the shared mailbox in your search results.
In the pane that appears at the right side of the browser, click Edit under "Email apps."
Place a checkmark in the box for "Outlook for desktop (MAPI)" ( see image below )
Save
Unless there's an additional problem, you should now be able to add the shared mailbox to Outlook for Mac.
(Example image)