Hi there, Keziah.

When converting a sales order into an Invoice in QuickBooks Online (QBO), the sales order number is automatically updated and used as the invoice number once it is already converted. This process ensures that the invoice number is unique and cannot already be in use.

To fix this, let's perform browser troubleshooting steps to determine if this is a browser-related issue.

First, log out of QuickBooks and then log back in to ensure the system refreshes properly. After that, reload the page and try adding a new product.

If the error persists, try accessing your account using a private or incognito window. This helps bypass potential issues caused by cached data, as an incognito session creates a fresh browsing environment without stored cookies or history.

Here are the keyboard shortcuts for opening an incognito or private window, depending on your browser:

Google Chrome: Press Ctrl + Shift + N

Mozilla Firefox: Press Ctrl + Shift + P

Microsoft Edge: Press Ctrl + Shift + N

Safari: Press Command + Shift + N

Lastly, consider switching to another supported browser for an optimized QuickBooks experience.
 

Please respond to this post if you have additional concerns related to QBO.