If Apple Pay does not show up in your checkout, check if all the conditions that need to be satisfied are indeed true. In order for Apple Pay to show up, the following needs to be in order:
- You need to be in a supported country or region
- It needs to be on an eligible device with the latest version of iOS, iPadOS, watchOS or macOS
- You need to have an Apple ID signed in to iCloud
More information can be found on About Apple Pay and Apple Pay on the web.