Hi @nbmoretto
Did you look at the FET data sheet for how to hook it up? The pins are from left to right with the heat-sink to the back: gate, drain, source, so gate gets the resistor to the Photon, drain goes to the solenoid/diode and source goes to the common ground connected to both the Photon and the 12V power supply.
Another good thing to try is to remove the end of the resistor connect to the Photon pin and make sure you can trigger the solenoid by touching it to +3.3V on the photon.