Alright, it's confirmed. I've mounted two other SSD's with High Sierra and Catalina pre-installed, both same issue. I have learned something that I did not know before: EFI lock will prevent hard drive replacement!

The keyboard connector is very close to the battery. Open the flap carefully on the keyboard connector and slide out the flex cable - then slide it back in and try to push it deep in the connector and close the flap again. Power on your macbook and test it. If that didn't work, remove the flex cable and inspect it for tears or damaged pins. Also inspect the connector on the motherboard for damage.

Hi Misha, Layer 2 is a acryl (or acryl-like) layer. The top pushes against the edge of the frame where the LCD rests on. The bottom pushes against the backlight bar. You can't simply put this layer down and move it into place like all other 4 layers. This acryl panel needs to be bent vertically. I recommend bending, pushing the top edge on the frame and then the bottom edgea gainst the backlight. Hope that answers your question. But I have a feeling you didn't mean the acryl panel layer although you refered to by mentioning “layer 2” which is the acryl panel. Edit: No, the second layer (the acryl panel) does not have any adhesive.