The easiest way to void an invoice in QuoteMachine, is to do so directly in Lightspeed Retail POS (R-series). You'll notice that when you use the void button in QuoteMachine, you will be directed to like speed retail to complete the void.
If you're looking to avoid a specific payment, that is explained right here.
Voiding a QuoteMachine invoice is a two step process.
First, select the invoice you wish to avoid. Use the void button in QuoteMachine, and you will be directed back to light Speed retail to void the invoice there.
Void the invoice in Lightspeed Retail.
Now, you will be able to equally void the invoice directly in QuoteMachine.