The game says a switch that should be open is stuck on.

"Switch is stuck On" message

Cause: A switch the game needs open, such as a coin or tilt switch, is closed, miswired or shorted.

Checks

• Find the named switch and check it is not held closed.
• Check its row and column wires are on and not shorted.

Fix

• Adjust or free the switch and repair any shorted wiring.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• "Check Switch ##" message
• "Check Switch #" message at start-up
• A whole switch column dead or all closed
• A whole switch row dead or all closed
• "Ground Short Row" message
• Using the switch edge test
• Check playfield switches with the switch edges test
• How the switch matrix reads a switch

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part One, Clay Harrell (pinrepair.com)