I don't think it's Endevor related -- it sounds like there's a model DSCB for a GDG on an SMS-managed volume. If your JCL doesn't have a GDG, contact your site support group that handles Endevor to have them clean up the error.
SMS is Storage Management Subsystem. It handles many of the functions of storage management for you; your site support group can set up rules that force certain data sets to be managed by SMS. Some system files cannot be managed by SMS as there could be side effects.
The error message makes it clear that a GDG is involved. You need to contact your site support group since the error message indicates that the model DSCB (in your example SYS1.MDSCB) is already part of the GDG definition and should not be.