If you've already entered a product or service on the invoice you can delete the line item by clicking on the trash icon. I'm here to help and guide you through.
Let's create a test invoice and try to delete the line item from there. Here's how:
For visual reference, I've attached a screenshot.
Just in case you need help customizing your invoice, I'm linking this article that will provide more information: Customize Invoices in QuickBooks Online (QBO).
Leave a comment how it goes. I'm always here to help.