If there is no invoice number on invoice, how can this be fixed?

I know what happened here, Jessica.

QuickBooks Online assigns invoice numbers automatically. It will only appear after saving the invoice.

Unless you turn on the Custom transaction numbers function, you can assign your own invoice number.

1. Click the Gear icon in the upper-right corner and select Account and Settings.
2. Select the Sales tab and go to the Sales form content section.
3. Mark the Custom transaction numbers box.
4. Click Save and Done.
   

Once you go back to the invoice, you can already add or change the invoice number.

Just get back to me if you have more invoice questions so we can address them. Have a great day!