You can't use AMEX as a payment method for Google Ads. Even though Google accept it on set up apparently AMEX send a "code" when a transaction takes place which they blame on Google for not integrating and asked ME to ask Google to integrate. Absolute joke. I was using Mastercard for 3 years without any issues what so ever. What an absolute waste of time. Customer service agents haven't got a clue either were more like "yeah we know sorry we will feedback that through". Here is a free tip for growth you might want to get AMEX accepted with Google.
