I’m having the same problem on my iPad Air. When I attempt to create an invoice from the Customers tab, and then tap (+) for a new invoice, I get the same pop up error.

A workaround that is working for me is to use the Invoices tab to generate the invoice. Going this route I have sent several invoices today with no issue. I assume QBO will fix the main issue soon. Seems to not be a caching issue since generating via the Invoices tab is successful.