cancel button shows up in the catalog request details page even the request is in completed or closed status
It is not the problem . Whether a catalog request with Completed status can be cancelled or not is based on your catalog system's setting in Catalog -> Configuration -> Request Management Configuration - > locate "Cancel Completed Requests" configurable setting :
If it is "Yes" , that means your catalog system allows to cancel the catalog request with "Completed" status .
If it is "No" , that means your catalog system does NOT allows to cancel the catalog request with "Completed" status .
"Cancel Completed Requests" is the newly-added configuration setting in 17.1 .