Can I use a P1806 Mosfet instead of a IRF520?
Well yes but only because neither of them are suitable for being driven by an Arduino.You need a logic level FET for that sort of thing.
Neither of them are Logic Lever MOSFETs so they're not ideal for use with Arduinos. But if you've already tested the IRF520 and it works for you then there's a good chance the STP1806 will too.Steve
*hit happens.Try IRL520
They are packaging the wrong MOS FET for direct Arduino driving.Maybe they ran out of IRL520s (L = logic).
Official Arduino Store = IRF520https://store.arduino.cc/genuino-starter-kit