Live demo unit re-flashed. got a security error

Hey all. I recently bought a live demo unit galaxy s8 plus. I tried to flash normal software on it thru odin. That passed fine with no problems. When I went to boot the phone after flashing, it throws up a error that says “Security Error; this phone has been flashed with unauthorized software & is locked. Call your mobile operator for additional support.”. At this point I dont care if it still has the demo software on it. I just want to have a working phone again. If you have any solutions please let me know. All help is appreciated. Thanks!