You shouldnt be having this problem, sorry to say. I recently was able to downgrade from Lollipop on my Note 4 to Kitkat while this error was at the bottom of my recovery screen. Although at the bottom it only says dm-verity verification failed and not the other line of need to check drk first. Try wiping your cache partition. Actually, check if you're using odin right. Did you download your old firmware from somewhere like sammobile firmwares? Check to see if you are running with adminstration rights when starting Odin.