"Payment not completed!" message using apple pay on Mac OS websites

Hi all,

When buying online I tend to use Apple pay as it is so easy. I use Safari using Mac OS 10.15.7.

Recently when I choose Apple Pay at checkout the payment confirmation will open on my iPhone 8+ (running iOS 14.4) but when I confirm with Touch ID it processes and then says "Payment not completed!" with a red exclamation mark and goes back to the checkout basket. This also happens when confirming on my apple watch (series 3).

I have looked online for fixes - I have tried deleting all cookies from safari. I turned my bluetooth off and on, and I have tried signing out of apple ID on my iPhone (& watch) and signing back in (which in effect removed my cards from apple pay). None of these fixed the problem.

When I opened the website on safari on my phone and paid with apple pay it worked fine. Apple pay for contactless payments still works fine.

Any help understanding the problem and fixing it would be greatly appreciated. Please let me know if more detail is needed. Thanks.