You are unable to log a comment on, or edit a ticket that has already been closed.
There is an option in Options Manager called "edit_inactive" which will prevent users from editing tickets that have been closed if the option is set to INSTALLED.
To allow users to be able to log a comment or edit a closed ticket, you must ensure that the "edit_inactive" option is NOT INSTALLED in Options Manager.
To do this, follow these steps:
1. Log into CA Service Desk Manager as an Administrator
2. Go to the Administration tab
3. In the tree on the left, navigate to Options Manager > Request-Change
4. Click on the option called "edit_inactive" and then hit the "Edit" button
5. Click the "Deinstall" button
6. Recycle CA Service Desk Manger services on all servers
You should now be able to edit inactive tickets in the system.