If you are unable to access a unified group mailbox, there could be several reasons why this is happening. Here are a few troubleshooting steps you can try:
Verify that you have permission to access the group mailbox: Make sure that you have been added as a member to the unified group and that you have permission to access the group mailbox. You can check this by going to the group's settings in the Microsoft 365 admin center.
Check if the group mailbox is hidden from the GAL: If the group mailbox is hidden from the Global Address List (GAL), you may not be able to see it in your email client. You can check this by going to the group's settings in the Microsoft 365 admin center and making sure that the "Hide from address lists" option is not selected.
Check if the group mailbox is properly licensed: Make sure that the group mailbox is properly licensed. If the group mailbox is unlicensed, you may not be able to access it. You can check this by going to the group's settings in the Microsoft 365 admin center and making sure that the appropriate license is assigned to the group.
Check your email client settings: Make sure that your email client is properly configured to access the group mailbox. Depending on your email client, you may need to add the group mailbox as a separate account or folder.
Try accessing the group mailbox through Outlook on the web: If you are still unable to access the group mailbox through your email client, try accessing it through Outlook on the web. This can help you determine if the issue is with your email client or with the group mailbox itself.
If none of these troubleshooting steps work, you may want to contact Microsoft support for further assistance.