Hi Clare303,
Welcome to the Apple Support Communities! Thanks for posting about your issue here. We'd be glad to help point you in the right direction. Do you know if you're able to use Apple Pay with other merchants aside from those two? If you're able to make other purchases, we'd recommend contacting the app developers for the apps you're having issues with for support. You can learn how to contact them by using the steps in How to contact an app developer.
If you're having issues with Apple Pay with every merchant you're attempting to purchase from, we'd recommend getting connected with an Apple Pay specialist at this time. You can reach them by heading over to Apple Pay Support and scrolling down to the "Tell us how we can help" section at the bottom.
Take care.