There is a hole on the back of the cutter that is a sensor to see if there is media or not. When you get the error, the vinyl is probably not laying flat on top of the hole because of the vinyl not feeding properly. Occasionally this will happen to me, but its usually just something up with the vinyl bunching up when the cutter feeds the vinyl backwards causing the vinyl to raise up in the back. I just smooth the vinyl flat and hit enter and the cutter will go back to cutting.
This is good advice. Most of the time when there is a measure/feeding issue with the Graphtec it is because the media is not being detected. Every once in a while on a CE-5000 you will get a "Missing Media" error, all you need to do is hit enter and the cutter will go along like nothing ever happened.