Could be a number of things, I’m an AASP. So ifixit process I am very unfamiliar with. But watching the videos I have an understanding of what might go wrong. Also it’s not uncommon for Apple screens to go bad, the first thing I would address would be, “was I wearing gloves and was I grounded?” This could cause damage to the parts if I wasn’t. I have had screens go bad when I wasn’t, it can be catastrophic or latent damage to the device or components. Next I would open the device and see if the cables were connected to the motherboard correctly, unplug and reset them. Before closing the device turn on the phone and mess with the screen. During this time I would take note of any bent cables and see if the orientation is correct or if the cables were damaged in the process of transferring them from the original screen to the new one. If all that isn’t the problem, then I would use a known working screen to see if the display was just bad. then give the customer the new display and do whatever process you have in place to return the non-working display to get money back so it’s not a total loss to you.