That’s correct, it’s fine to create invoice. If there were an open invoice, you’d have the option to attach the ticket to it. This is useful when grouping multiple tickets under a single invoice.
For example, I often create an invoice titled “[Month Name] Support and add tickets to it throughout the month. It’s a great way to prevent microbilling a client for every ticket.
Technically that little warning message is a little mis-worded and should probably be change or maybe it doesn't even need to be there in the first place, but if there's open invoices allow to select from a list as well as creating a new invoice.
