Black thick line on left side of MacBook Air 2020 13 inch

Hi I got this I think preowned MacBook Air and the issue with it is after since I updated it to the latest update and since then this black line has appeared and it’s color lightens and darkens depending on the lock screen and I contacted apple and they told me that this would be a 300$ repair but I’ve done nothing and I don’t know if this is a problem with the screen and it’s ink inside the display or whatever is inside the MacBook and it has just said it has had an error, but I will list photos below..can someone tell me what this is and how or what can I do to fix it? The pictures have both sides and the difference over the course of 4 days