I am from Canada and I want to know how my customer can pay me with american express card. They say they cannot pay me with american express card.

We can submit a request to add an American Express account as one of the payment methods, Topmoversinc.
 

You can follow the steps below for the process: 

1. Sign in to the merchant service center.
2. From the Account column, select Account Profile.
   
   
    
3. From the Payment Methods, click Add American Express.
   
   
    
4. Hit Submit.

The process may take up to 2 business days. Once approved, you should see the American Express logo when processing customer payments and add it as your payment option. 
 

Please see this article for more information: Receive and process payments in QuickBooks Online.

Moreover, you may visit this article to get answers to common questions about QuickBooks payments: Frequently asked Questions.

If you have additional questions or concerns about processing payments in QuickBooks, please leave a reply below. I'll be around to provide the necessary help.