This File Error is given by our Prepress team if the crop marks in your file are too close to the artwork or otherwise placed incorrectly.
Crop marks should be set 0.1" away from your artwork (the trim line).
Please do not include any border for the trim line (or die-line) on your artwork.
We have templates available for most products to help you set up your artwork correctly which can be found on product pages or under the Templates heading on our website.
You may also want to view our sample of a press-ready business card file.
The animation below provides a reference example of how crop marks are used to produce your final product: