If you've found an invoice with no transaction number assigned, this is likely a rare occurrence. If you want to manually assign a transaction number to the invoice follow the steps below.
Please Note: It is important that you do not create a duplicate transaction number during this process. See below for more information.
Manually Add Transaction Number
1. Go to your QuickBooks Online account and take note of the most recent invoice transaction number.
2. Enable custom transaction numbers:
Click on the gear icon (top right) and select "Account and Settings"
Go to the "Sales" tab.
In the "Sales Form Content" section, change the "Custom Transaction Numbers" setting to ON.
3. Find, open, and adjust the invoice(s) that are missing a transaction number.
4. Create a new invoice and add a transaction number that is one number higher than the most recent number (noted in step #1).
5. Turn custom transaction numbers back off - see step #2 above, but change the setting to OFF.
6. Create a new invoice and make sure that the number starts in the correct place.
More Information
Generally we do not recommend custom transaction numbers. To learn more about why, please see the Custom Transaction Numbers in QuickBooks article.