Good day, @matt244. I'm here to provide some insights why QuickBooks Online (QBO) prompts you that you've already used an invoice number when creating invoices.

By default, you would only receive this prompt when two or more invoices have the same invoice number. This prompt only occurs once, and you can proceed sequentially entering invoice numbers for new invoices. It would help us if you could provide a screenshot or more details of the issue you've encountered.

For now, I recommend searching for the invoice transaction using the invoice number at issue to check if an invoice transaction exists. If you've found transactions with the same invoice numbers, you may perform the necessary actions of editing or deleting them. Also, I suggest temporarily turning off Custom transaction numbers in your sales settings and running a sample invoice to help us further isolate the problem. If you encounter the same, I suggest performing browser troubleshooting steps.

This article on matching bank deposits in QuickBooks by combining multiple transactions into a single record might come in handy for you in the future. I'm adding this as a reference: Record and make bank deposits in QuickBooks Online.

You're always welcome in the QuickBooks Community should you have further updates on the issue of invoice numbers. Take care.