Hi, we are testing Apple Pay in the sandbox, but after selecting a test card from Apple Pay and returning to our site, it simply displays “Apple Pay is not available for this website”.
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Additional Documentation
Square Sandbox
Supported Payment Methods by Country
Test in the Sandbox
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Apple doesn’t allow Sandbox test credit card values. You must use a valid credit card from your Apple Pay Wallet. The card isn’t charged for test payments as long as you’re testing in the Square Sandbox. 
Hi Bryan! Thanks for the replying
What about the test card provided by Apple Pay: Test Cards for Apps and the Web?
We tried it with the In-App Payments SDK in the sandbox environment, and it worked without an issue. So, we are wondering if it can be used to test with the web payment SDK, too.
With our Web Payments SDK you must use a valid credit card from your Apple Pay Wallet according to the team. The card isn’t charged for test payments as long as you’re testing in the Square Sandbox.