Quoted from Kurisu:

I'm pretty sure the On Ramp Bypass is right under the map and just above the bumpers. There's probably nothing wrong with it. This switch almost never gets used and that's why you're seeing an error report.
To "fix" it, you need to plunge the ball really lightly so that it goes through the lane right before the skill shot ramp. If you do it right you'll score 10K points. Turn the machine off and on again to confirm the error cleared.
No one would ever plunge the ball that lightly in a real game (the point of the skill shot is to hit the 100K), and consequently it rarely gets used and every so often you get the error at start up.