Take the shell off and check all the wiring to the reverse board. Then move the reverse switch to make sure it is not locking the board in neutral.

The switch can be bad. Unsolder the wires to the switch and test the board, then touch the wires together and test again. That should change the state of the board if it is functioning. If you get no response, the board is bad. Replace the switch and the board at the same time.

Larry