Hello everyone.

I have the troubleshooting steps you need to resolve the error encountered when adding a new invoice number. Let me guide you through the process so you can create invoices with the appropriate number.

In QuickBooks Online, you'll receive the error "This invoice number was already used. Try a different one to continue." once you've entered a duplicate one. However, we currently don't have reported cases where you'll get this error despite entering a new one. In the meantime, you can turn off Custom transaction numbers to isolate the issue.

However, if the issue persists, let's verify if the cause of this behavior is a corrupted cache file. To begin with, let's utilize a private window to prevent these outdated data while running the program. You can proceed to these keyboard shortcuts depending on your browser:

• Google Chrome: Ctrl + Shift + N
• Safari 11 or newer: ⌘ + Shift + N
• Mozilla Firefox: Shift + Ctrl + P
• Microsoft Edge: Ctrl + Shift + N

Once you've added a new invoice number without any issue, you can clear the cache from your regular browser to refresh the website's preferences. You might as well use a supported one for the best and most secure experience.

On the other hand, consider scanning this material once you're ready to receive customer payments: Record invoice payments in QuickBooks Online.

Please update us in the thread if you have other queries when adding a custom transaction number or any related concerns in QuickBooks Online. We'll be available to offer further assistance.