The Average Days to Pay report is only available in the desktop version. As a workaround in QuickBooks Online, you can open the Invoices and Received Payments report. Then, export it to Excel so you can create formulas to get the difference of the dates of the invoices and their payments. Then, get the average of the differences to get the Average Days to Pay per customer.
The workaround I suggested is ideal if you have a few transactions. However, if you have a lot of them, I'd suggest finding third-party apps that can extract the average days to pay per customer.
If there's anything that I can help you further, feel free to go back to this thread.