You're gonna want to re-wire from the panel to the receptacle.
Don't put a 20 amp receptacle on a 30 amp breaker.
Don't pull the 30 amp breaker out and replace it with a 20 amp breaker and install the receptacle on the wiring that was meant for 30 amps. The breaker protects the machine and the wiring by (hopefully) tripping when the current load (and thus the heat in the wires) becomes excessive. If the breaker is oversized, then the 30 amp breaker won't trip until the load is 50% more than what is called for to protect the machine. A machine that is "supposed" to run drawing a maximum (startup) 15 amps will trip the 20 amp breaker at the point of a 33 1/3% overload (or less, hopefully). If it's on a 30 amp breaker, it won't trip until it's drawing 100% more than its designed current draw. The machine or the wiring from the machine to the receptacle will burn up first.
If the wiring is oversized, then the 20 amp breaker won't trip "when it's supposed to" because the heavier gauge wire won't heat up as much/as fast as the smaller gauge wire. This leaves the wiring from the machine to the receptacle as the "weak point" and it will burn up first (before the breaker trips).
A 20 amp 220-240v circuit calls for 12 gauge wire assuming copper is used.
A 30 amp 220-240v circuit calls for 10 gauge (or larger) wire, again assuming copper is used.