I have tried to contact QB via chat but it doesn't work. I want to issue a credit against an invoice , is this possible?

Hey sales61,

Thanks for your post and welcome to the QuickBooks Online Community!

You can do this on any of our small business packages (Simple Start, Essentials, and Plus) by following the steps below:

1) Go to the + icon and select credit note

2) Enter the customer, the date the credit was issued, and the line details with the product/service and amount

3) Save and close the credit note and go to the customer 

4) On the invoice, select 'Receive payment' - on the receive payment screen tick the invoice under 'Outstanding transactions' against the credit note under 'Credits'

This will apply the credit note as a full or partial payment against the invoice.