Domain Validation error/message for Apple pay / Stripe

My client is having error message when apple pay is used. I’ve looked everywhere and even contacted Stripe and they are saying I need to upload a validation file to my site but there is no clear guide as to what this file is or how I get it from Apple Pay? Has any of the solved this or able to point me in the right direction?

I’ve checked all my settings and Apple Pay is clicked on as per one guide I’ve found.

Appreciate your help in advance.