Please refer the below note
Cancel Button Grayed In Process Message Window After Click On Book Order Button (https://mosemp.us.oracle.com/epmos/faces/ui/km/DocumentDisplay.jspx?id=412683.1Doc ID 412683.1)
This applies to your case
There is an ER to have the CANCEL button removed from the Process Messages form in this case. so the users are not confused by the new (correct) functionality.
An Enhancement Request (ER) 5850065 is logged with Oracle Development. This ER will be reviewed and considered by Development for a future release. It will remain open until it is either incorporated into the product, or it is deemed not feasible. Progress on the ER can be monitored via MyOracleSupport by following these steps: