Apple Pay offers an easy, secure, and private way to pay on iPhone, iPad, Apple Watch, and Mac without a physical card or cash.

As explained in this support article from Apple, in order to use Apple Pay, you need to:

You can use Apple Pay to buy things in stores, within apps, and on the web in Safari.

Kindly note that Apple Pay is only available on eligible Apple devices and on the web using the Safari browser. 

If you use a non-eligible device or another browser (i.e Google Chrome or Microsoft Edge), the Apple Pay button won't render, even if Apple Pay is available on the app or website you are using.