Welcome to the Community, Zanocontractingl.
In QuickBooks Online (QBO), you must sign up for QuickBooks Payments to accept credit card payments or other online payment methods. Once set up, you can instantly receive customer payments.
Moreover, if an error occurs upon accepting payments from your customers, kindly, return to this thread and communicate further details. We'll ensure to look into it and provide adequate solutions promptly.
Furthermore, various reasons affect the acceptance of payments from your customers, such as the payment amount exceeding the limit set by the card issuer or bank. I also recommend your customer reach out to their bank regarding this matter.
Additionally, I suggest browsing this article to assist your customers if they cannot pay your sent invoices in QBO: What to do if your customers can't pay their invoices online.
Keep me posted if you need assistance receiving Amex payments in QBO, Zanocontractingl. I’ll jump right back in to assist further. Have a great day ahead!