What permission is required to change the "A release is scheduled for this iteration" checkbox?

The Scrum Master role already has the Modify the iteration structure permission, with Modify the collection of timelines and Modify the structure of iterations permissions as well.  So don't believe that is the correct one.  If it is, then we have other issues.

Thanks, Christian, for asking about the error message.  My developer only told me it was a permissions problem.  I used one of our testing IDs to recreate the problem myself so I could see the error message and when you expand the error message it tells you that it needs the "Modify project area properties" permission.  Not sure why he didn't relay that part to me.  I tested it out and was able to modify it once I gave the Scrum Master role that permission.  Now I just need to figure out what other things can be changed when given that roll before I assign the Scrum Master role that permission on our production server.  Thanks!