This article explains why checklist tasks may not be able to be marked as complete.
When you hover over a checkbox on a transaction's checklist, the checkbox is disabled and you can't check the task off.
Only users with permission to fully manage checklists may mark tasks as complete. Agents are not typically granted permission to mark tasks as complete. If you're unsure about whether you have been granted the permission to check off tasks, you should check with your admin.