If you click on a ticket URL and get an error message that says "You do not have permission to view that ticket" it means exactly that. :)
Every mailbox has permission settings that control who can see the tickets in that box. Only admins of the account can access these permissions under Settings > Mailboxes down at the bottom of the settings page.
Alternatively, if full access cannot be given for whatever reason, then the ticket can be moved to a mailbox you are able to access.