The flight controller and ESC wiring harness is firmly plugged in.

[Secure the 8-pin cable] Make sure the 8-pin Plug between the flight controller and the ESC is firmly plugged in. The connector is divided into two stages: if you don't push hard enough, it may only be inserted into the first stage. You need to apply a bit more force to fully insert it into the second stage to ensure a proper connection.

If you suspect there is a poor connection inside the cable, you can try swapping the two ends of the cable — that is, connect the end that was originally plugged into the flight controller to the ESC side instead.