When you see dark lines on a print of a solid color  (see example below) it is due to the print head being too close to the material (head striking).  This can occur at any part of the material but usually occurs on the leading edge or in the middle of the product.   In order to fix this issue, go to your printer and go to the paper menu.  In the paper menu click on “Paper Detail Set” then inside of paper detail set choose your paper type you are loaded with.  Choose “Head Height” then you will see a check next to the setting it is currently defaulting to.  Change that setting to the next level up and reprint.  If it still occurs keep moving that setting up until the head striking disappears.   (note: any changes you make to the paper detail set will not change until you change them back.  Also, increasing your “vacuum strength” may also help eliminate this in combination with a head height adjustment.  You can adjust to a stronger “vacuum strength” in the Paper Detail Set under the paper you are using)