Apple Pay not coming up when trying to use it

• Where are you attempting to use it?
• Not all retailers support Apple Pay.
• When used at a retailer terminal that accepts Apple Pay the phone needs to be placed very close to the NCF symbol on the terminal. That will prompt for Face ID to authenticate with the double click of the side button then you have to place it back on the terminal. Or you can authenticate first by double clicking the side button then place it on the terminal just the one time.

Open the Wallet app and select the card, then double click the side button. Does it show the icon to place near a terminal to complete a transaction?

If you want that card to always be the Default card that shows up when using Apple Pay, then you can change that here.

Change or remove the payment cards that you use with Apple Pay - Apple Support

Double click your side button. Apple Pay activates and you see your default payment method and loyalty cards, passes, etc. at the bottom. Tap and swipe the default card down and all your other payment methods will appear. Tap on preferred payment method and it’s now selected for active payment when you double click to verify and complete payment.