Diagnosis
The igniter electrode isn't sparking near that burner's gas port. The wire leading to it probably got disconnected or misaligned during assembly, or the electrode tip is too far from the burner.
How to Fix It
Lift the griddle top off the cart (get a helper, it's heavy). Look underneath at the back of the control panel. You'll see a thin, insulated wire running to each burner. Trace the wire for the dead burner—its metal electrode tip should be positioned about 1/8 inch from the burner's metal port. Reconnect any loose wire and adjust the bracket so the spark gap is correct. Reassemble and try again.
If That Didn't Work
With the top off, have someone press the igniter button for that burner while you watch underneath in dim light. You should see a blue spark jump at the electrode. If not, and the wire is connected, the igniter module inside the control panel might be faulty for that circuit.
Heads Up
You can still light the burner manually with a long-reach lighter if the igniter fails—just hold the flame at the port and turn the gas knob. It's a workaround, but it confirms gas is flowing and the issue is purely electrical ignition.