Diagnosis
This is a clogged printhead nozzle, but the standard 'Clean' from the software often isn't enough. Ink has dried in the microscopic nozzles.
How to Fix It
You need to run the *deep* cleaning utility. On the printer's control panel, navigate: Setup > Tools > Print Quality Tools > Clean Printheads. This uses more ink and takes longer. Do it once, then print a nozzle check pattern (also in Print Quality Tools) to see if the lines are gone. If improved but not perfect, wait an hour and run a second deep clean.
If That Didn't Work
For a stubborn clog, you might need a 'head soak'. Power off the printer. Place a folded paper towel dampened with a few drops of distilled water in the service station area (where the printhead parks on the right). Manually move the printhead over to park it on the damp towel. Leave it for 8-12 hours, then power on and run a deep clean.
Heads Up
Don't run more than two or three deep cleans in a row—you'll waste a huge amount of ink. If the issue persists after that, the printhead itself might be failing and need replacement.