how to fix pcb to qualcast lawnmower?

Hi, could you clarify what you mean about PCB connections being correct? The design of these boards will have specific wiring connectors on the board that don't really allow anything but the correct connections if using the original wiring looms.
I don't know the details of your PCB, but may be worth checking for any swollen/blown capacitors on the board if they are present.
Have you run a multimeter across the L & N terminals at the motor to determine if the power is getting that far? I'd recommend removing the blade whilst working live, just in case.

Rupert
June 2020