This is because they are using AmEx and your paypal settings aren't configured to accept AmEx... you can update that on your paypal account.
You can see the error message from Paypal on the page you have attached: "Credit card type unsupported"
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.