Lift up the display cable and peel off any remaining coating holding it to the motherboard.

During reassembly:

Inspect the head of the cable and the ZIF connectors for any remaining coating that could prevent a good connection.

Peel off the coating—heating the cable and the ZIF connector when the coating becomes too brittle.

It doesn't have to look pretty! You just need to remove enough coating to have the cable sit properly in the connector.