How do I fix a screen problem on my MacBook Air?

Usually vertical lines/bars are caused by a hardware issue with the Display Assembly.

You can confirm by taking a screenshot. View the screenshot on another device, or if you view it on the same laptop, then move the picture to the left so it does not overlap that section of the screen. If the anomaly does not appear in the screenshot, then you have confirmed a hardware issue with the built in Display Assembly.