Occasionally when copying and/or scanning, you may find that a solid line appears down the page for apparently no reason at all. However, if you are seeing a solid line down the page, then this could indicate an issue with the machine that you are using, and you may need to take further action.
Generally speaking, if there is a clear solid line down the page that you have copied/scanned, this tends to be down to a fault in the cartridge or an issue with the paper. For example, if the paper is damp or the wrong type set in the print driver, this issue may arise.
If you are using a colour laser/LED printer, you check the condition of the appropriate colour cartridge and the transfer rollers for damage. If, on the other hand it’s a mono laser with a black line, it may be worth taking the image drum out (seated under the toner cartridge) and checking for any obvious signs of physical damage. The line on the page should match up with scoring on the drum.
If you have a vertical line down the page when copying using the automatic document feeder, there will be some contamination on the scanning LED’s. You can clear this by lifting the scanner lid and cleaning the glass strip at the back of the glass platen.
And then fingers crossed, this should eradicate the problem! However, if after all of this, the issue persists, I would strongly advise contacting the manufacturer of your machine for some further advice.