AA21 Didn’t pay prefund

The sender did not pay the EntryPoint

The sender did not have enough to prefund the EntryPoint for the user operation. If you are using a paymaster, the paymasterAndData field is likely not set. If you aren't using a paymaster, the address of the sender does not have enough gas token. After the user operation is executed, the remainder of the prefund is credited back to the sender.