If you have Custom transaction numbers turned on for your account, then QuickBooks Online may leave that field blank unless you’ve specified a name or prefix.
When the Custom transaction numbers option is turned on, QuickBooks Online will not automatically assign a plain invoice number to the invoice.
Instead, QuickBooks Online will number from what you specify, like ABC-1000. So the next invoice will be ABC-1001. If you then turn off the custom transaction number setting, then QuickBooks Online will continue the count. The next invoice that QuickBooks Online creates will be ABC-1002.
To change the settings, go to the Gear Icon > Account and Settings and Sales.
Here’s how the invoice will look like with Customer transactions numbers turned on:
Here’s the invoice with the setting off:
Notice how the Invoice no. field is gone.