DIAGNOSIS That's a dirty or scratched primary charge roller inside the toner cartridge. The line repeats because the drum inside the cartridge is rotating.
FIX The defect is in the cartridge, not the printer. Remove the toner cartridge. Rock it gently side-to-side to redistribute the toner. This can temporarily clear a line caused by debris. Reinstall and print a few pages. If the line remains, replace the toner cartridge.
IF THAT DIDN'T WORK If a new cartridge shows the same line in the exact same place on the page, the problem is in the printer. The transfer roller (inside, under the cartridge) is dirty. Power off, unplug, and clean the long, black rubber transfer roller with a lint-free cloth dampened with distilled water.
HEADS UP Do not use alcohol or solvents on the transfer roller. They degrade the rubber. Never expose the cartridge's green photosensitive drum to light for more than a few minutes. It will be permanently damaged.