What is so1?
Good day, vidhya_dessai.
If "so1" is a third-party, you'll want to contact them so they can walk you through on how to link it to QuickBooks. They can also assist you on how to create and import an invoice.
However, if you're referring to a sales order, you can click on the Create an invoice button. That way, it'll be converted into an invoice. Open the sales order to see this option.
You'll want t check this article about Accounts Receivable workflow.
Let me know if you're referring to something else. I'll be right here to help.