first of all, your domain needs to be prepared, the domain have to host the apple-developer-merchantid-domain-association file and the merchant has to be registered using the API or in the developer portal https://developer.apple.com/documentation/applepaywebmerchantregistrationapi/preparing-merchant-domains-for-verification after that, the domain must match the domain registered against Apple Pay, when you click on the Apple Pay button, the SDK validates the merchant and the domain of the session with the domain of the browser, it must match exactly