If you want to process an invoice with a total amount of $0, make sure these highlighted fields match your invoice:
The COMP payment method will be used exclusively for complimentary invoices, with a zero ($0.00) total. If a different payment method is selected for transactions with zero amount, the system will show this error: "This transaction cannot be processed with the payment method selected" and vice versa.
The invoice after payment will automatically move to Paid status.
PS: We are aware some customers are getting the same error message mentioned above when saving a working invoice with payment method different than COMP. Our team is currently working on a fix for this issue.