Apple's System Integrity Protection feature, introduced in macOS 10.11, requires that you add a trusted NetBoot server to a computer before you can use a policy or Casper Remote to boot that computer to a NetBoot image. If you attempt to boot to a NetBoot image on a macOS 10.11 computer that does not trust the NetBoot server, you will receive the "Could not set boot device property/Can't set EFI" error message and the boot will not be successful. You only need to add a trusted NetBoot Server once per computer.
For detailed instructions on how to add a trusted NetBoot server, see Apple's Prepare for NetBoot, NetInstall, and NetRestore requirements in OS X El Capitan Knowledge Base article.
After you have successfully added a trusted NetBoot server to the target computer, you can boot that computer to a NetBoot image using a policy or Casper Remote. For detailed instructions on how to boot to a NetBoot image, see the "Booting Computers to NetBoot Images" section of the Casper Suite Administrator's Guide.