QuickBooks Online (QBO) automatically calculates sales tax amount based on the state where you have nexus and are registered to collect sales tax, physical address of your business and on the forms, and the product mapping you've associated with the item you're selling.
Let's also ensure you've added the customer's shipping address. I'll guide you how.
Go to Sales and select Customers.
Click on the customer's name and click on Edit.
Review and make sure you've added an address in the Shipping address field.