If you are not able to view a ticket and if it throws an error that says The ticket is either not there or you don't have permission then the cause can be either of the below:

1) That particular ticket would have gotten permanently deleted from the Helpdesk.

2) The ticket you are trying to view must be out of your scope, that is, you must have a Restricted ticket access. You could contact your Account Admin to know what scope you are assigned to. To know more about roles and scope refer to this link.