I dont have access to the work system now but, lookign at the video when you are putting the grip edge of the plate in the clamp it is coming up with the error and not closing the clamp? if this is the case most likley it is an error sugeting the plate thickness is too much or too little, usually it will close the clamp and then open again and throw an error if this is the case?
If this is what it is doing you have bolts going accross the plate clamp either lossen them (a very little bit at a time) and see if it makes a difference, if not tright to tighten them.
As i said i cant check this is what the error is as i am at home but looking at the video i think this must be the case, i dont run a 300 anymore so this is all from memory.