20 amp breaker with nothing plugged in & will not stay set. What would be the cause?

Is it an arc-fault breaker?

Question:

Are you using this breaker just to fill a position? Why?

When you say nothing plugged in, do you mean that there is no hot conductor attached to the breaker or do you mean that there is nothing connected to the various outlets for this circuit?

Normally, if a breaker trips immediately when it is placed on the circuit, there is either a direct short on the circuit or the breaker is defective.

My suspicion is the circuit. Have you tried swapping the breaker with a known good breaker?