The machine is trying to get that score reel to zero. It will keep trying until that happens.
With power off, pull the reel and index it with your finger until it is at zero, then power on and try to start a game.
Look at some videos on how to clean/rebuild stepper units - a score reel is considered a stepper unit. You may end up having to clean several of them.
Good Luck, and keep the thread informed of progress.