If the line is vertical, through the entire page length-wise, the contamination on the glass might be an option.
If the line is on the bottom of the page, width-wise, the line is likely the shadow of the end of the page. I would highly recommend making sure the size of the paper is selected correctly. When you hit copy, you can go into the settings and verify that the paper is not accidentally set to A4 when it should be Letter size. You can also reduce/enlarge which will clip more.