I am trying to create an invoice but when in try to save it is error invoice number already assigned but it doesn’t show the number for me to back check

I have the steps you can take to resolve your concerns about assigning invoice numbers in QuickBooks Online (QBO), swiftflow.

Since you’re certain that the transaction number isn’t assigned to any of your existing invoices, I advise taking troubleshooting measures to fix frequent issues that slow down the application's performance. 

We are able to follow the instructions in a private window. The private window is a helpful tool for troubleshooting browser-related difficulties because it won't preserve any history. The following are the keyboard shortcuts:

• Google Chrome: press Ctrl + Shift + N
• Mozilla Firefox: press Ctrl + Shift + P
• Safari: press Command + Shift + N

If a private browser shows the correct time posting of the trips, switch back to your default browser and delete its cache. The best way to give your browser a fresh start is to clear the cache or cookies. If not, using one of the supported browsers can also be a smart choice.

Once resolved, you can mark this paid once you receive the actual payment for this one. I have an article here where you can refer to keep your records accurate: Record invoice payments in QuickBooks Online.

Moreover, you can utilize various financial reports to get a snapshot of your business's financial growth. I also added a resource where you can refer to the complete guidelines for customizing it:

• Run reports in QuickBooks Online
• Customise reports in QuickBooks Online

Feel free to mention me if you have more concerns about invoices. I’ll be around to lend a hand for help.