Chances are that you're using a Cricut Explore Air 2 and glossy Vinyl or HTV or you're in a too bright or too dark space. You get this error when the Cricut's sensor can't clearly scan the black box around your design. If you've already adjusted the lighting in your room or the lighting is pretty standard, you're going to want to reduce the glare on the registration lines. Some people suggest applying matte scotch tape to the registration lines but I've got a less wasteful method. Use Elmer's glue! Here's a video showing how I do that! Basically, the white Elmer's glue quickly dries and takes the shine off the registration lines!