That's a common issue with the Sp300s. The obvious first thing to look at is that the pinch rollers are actually in the proper position. If they are and it still gives you that error, you need to change the cut tool trailing cable. I have them here if you need it.
Had a similar issue with mine a while back. Left pinch roller error everytime a new media was loaded to get a media length reading.
Cleaned the encoder strip (linear scale), and it worked. Ended up flipping this after a while when it came back then changed it when the new one arrived.
Fixed my issue.