Thanks for coming to the Community to get answers to your QuickBooks questions. I'd be happy to help clear this up for you.
QuickBooks Online Payments is an additional feature you can add to your subscription. It's a great tool to help you and your customers quickly end out your sales. I know how important it is to allow customers to pay with a variety of credit cards. At this time, customers can pay with Visa, MasterCard, Discover, and Apple Pay.
You're not the first user to request access for American Express with QuickBooks Payments. I'm going to pass this feedback on to the product development team, as well as our payments team. I'd encourage you to submit feedback as well. You can do this directly within your QBO account by clicking the Gear icon > Feedback.