Hi, @Mattisse99
Thanks for posting to the forums. I hope you’re doing well!
The ability to accept different payment options depends on the payment gateway you are using. Are you currently using Shopify Payments?
If so, you can disable American Express by heading to Settings > Payments in your Shopify Admin. From here, select Manage then deselect American Express from the payment options.
I hope that helps! In the meantime, I would love if you could provide the domain for your store to see what additional support I can provide.