As you are looking at the genset, on the left side there is a connector with 8 small wires in it.
Unplug this connector, and try to start the gen with the switch on the gen itself.
Will it crank now?
If not, here is what you need to do next:
Remember, as you are facing the genset, the front panel is actually the panel to your right - the large green panel.
Some gensets have a door in the front panel. If yours does, remove that door, if you have access.
If it does not have a door, the panel will need to be removed.
Inside, at the starter, test the large battery cable to make sure you have 12V there.
If you do not, the cable is broken between there and where it connects to the genset.
If you have good voltage, there is a small wire attached to the starter solenoid. This wire will be labeled K1-S. Test this wire for 12V while pressing the start switch.
Is 12V present there?