There is a black line in the middle of the screen, about two centime

Hi Tarik,

Without seeing your screen I'd have to say that the most likely scenario here is that the LCD is damaged or defective and probably needs to be replaced. A bad cable could theoretically cause that sort of problem, but since the cable is integrated with the screen, the result is the same; you'll still have to replace the LCD.

I can try to confirm that if we can see a picture of your screen, but I'm fairly confident about this diagnosis. Here's a guide to adding a picture to your question.

Adding images to an existing question - iFixit Repair Guide

Unfortunately if you do end up replacing the screen iFixit doesn't have a guide for you, but the good news is that there's a French site called SOSav that does. The translation has a couple of idiosyncrasies like calling batteries "drums" and flex cables "tablecloths", but otherwise the guides are well written and complete.

LCD screen iPad 7 (2019) repair - Free guide - SOSav