"Record Not found" message shows up when trying to access to a record from Ticket page / Ticket Conversation widget
This is due to the Read ACL restriction for the record you are accessing, Please create a table level read ACL access for the record you are accessing.
For Example, https://<yourInstance>.service-now.com/sp?id=ticket&table=sc_req_item&sys_id=165c2c0f0f71c700a34f9acae1050e3e
"table" - parameter indicates the table you are displaying the record
"sys_id" - Unique sys_id for the accessing the record
Please also review KB0639323 for additional troubleshooting information