Well, I'm sure you know this but the manual says:
ERR51 PINCH POS*
Pinch rollers are not properly set on the grit roller.
Set the pinch rollers on the grit roller.
If you have already insured that they are in fact positioned on the grit rollers then I would suspect there is a problem in the plotter sensing that last pinch roller. Make sure that it is clean and the sensor is not obscured. I think that may be the little white mark on the flat piece just above the roller.