"Disc not suitable for this system
" means you don't have a valid Saturn game inside your Saturn, meaning you probably have a bad burn since your Saturn is not detecting it as a Saturn game. Solution: re-burn and if problem persists, find a different copy of the ISO, else forget this game and move on to a different game.
"Game disc not suitable for this system
" means you do have valid Saturn game inside your Saturn, but the country/region code does not match that of your Saturn. Solution: patch the ISO to the correct country/region code with the Saturn Country Code Changer (a.k.a. SatConv)
found in the Misc.
section and burn again. If you don't know the correct country/region code, take a guess until you get it right (CD-R's are cheap).