buck converter doesn't work
Can you please help me and tell me what's wrong with circuit?


Your MOSFET is an N channel type and you have put it in the circuit as a source follower. Your gate driver can produce no more than 15 volts to drive the gate hence, the output from the source will be several volts below this figure and fairly load dependent.

To properly turn on that MOSFET you need your gate voltage to rise above the positive rail of your main power supply of 60 volts. You are nowhere near this happening with your circuit.

You should connect Vee of the HCPL3120 to the incoming positive rail and use a 15 volt supply that is floating and can be connected to the positive rail of 60 volts thus making a driver rail of 75 volts - that will then allow the gate to be properly driven.