Unable to add invoice number by default

Hello there, @shruti.

Welcome aboard to the Community. Let me help you add invoice numbers in QuickBooks Online.

You can configure your settings to view and change your transaction numbers in the system. Once you enable the option, it'll add a separate field for you to input the invoice number.

Please follow the detailed steps below:

1. Click the Gear icon in the upper-right corner.
2. Select Account and Settings.
3. Go to Sales.
4. Below Sales form content, enable Custom transaction numbers. 
5. Press Save, then hit Done. 

For additional reference, you can use the following articles to learn more about invoices, like how to enter them and make payments in QuickBooks:

• Create invoices in QuickBooks Online.
• Record invoice payments in QuickBooks Online

Drop me a comment below if you have any other questions. I'll be happy to help you some more.