Can't add shared mailboxes

1. Check Permissions: Make sure you have been granted access to the shared mailbox. Contact your administrator to make sure the permissions have been correctly assigned in Microsoft 365 Administrator Center and check the permission settings of the shared mailbox.
2. Manually add a shared mailbox: Open Outlook, click “File” > “Account Settings” > “Account Settings”.
Select your primary mailbox account and click “Change”.
In “More Settings”, select the “Advanced” tab.
Click “Add”, enter the name of your shared mailbox and click “OK”.
3. Using Outlook Web App (OWA): Login to Outlook Web App. 
Right click on the main mailbox name in the folder list on the left and select “Add Shared Folder”.
Enter the name of the shared mailbox and click “Add”. 
4. Check the auto-mapping settings: If the shared mailbox is not automatically mapped to your account, auto-mapping may be disabled.
Run the following command in Exchange Online PowerShell to make sure auto-mapping is enabled:
powershell
Add-MailboxPermission -Identity email address removed for privacy reasons -User email address removed for privacy reasons -AccessRights FullAccess -AutoMapping $true
5. Update Outlook: Open Outlook, click File > Office Accounts > Update Options > Update Now. “Update Now” to make sure Outlook is the latest version. 
6. Repair Outlook Profile: Open “Control Panel” > “Mail” > “Show Profiles”.
Select your profile and click “Properties” > “Email Accounts”.
Check if the shared mailbox is listed. If not, try adding it again.
7. Check Cache Mode: Open Outlook, click “File” > “Account Settings” > “Account Settings”.
Select your account and click “Change”.
Make sure “Use Cached Exchange Mode” is enabled.