Trying to Backout an already EXECUTED package and getting an INVALID SELECTION for the dialog.
Release : 18.0 18.1
Component : CA Endevor Software Change Manager
If the package is in Executed status, check to see if there is anything to be backed out for the Package.
Go to the Package Display screen for the package and check if the option B - Display Data Set Backout Info highlighted or not.
If the the Backout Info option is not highlighted, there is nothing in the package to be backed out.