Go to Settings > Accessibility > Touch > Back Tap. Choose Double Tap or Triple Tap, then choose an action.
Allow access to passes when iPhone is locked
Go to Settings > Face ID & Passcode or Touch ID & Passcode, then turn on Wallet (below Allow Access When Locked).
On an iPhone with Face ID, you can also use AssistiveTouch or use Switch Control to confirm payments with Face ID instead of double-clicking the side button.
Turn on Back Tap
Check that you have the latest version of iOS on your iPhone 8 or later. Go to Settings > Accessibility > Touch, and tap Back Tap. Tap Double Tap or Triple Tap and choose an action. Double or triple tap on the back of your iPhone to trigger the action you set.
Apple Pay is built into iPhone, Apple Watch, Mac, iPad, and Apple Vision Pro. To get started on iPhone, open the Wallet app and tap the plus symbol. Then add a credit or debit card by tapping the back of your iPhone with your eligible card. You'll have the option to add your card to your other devices at the same time.
If your iPhone has Face ID, double-click the side button. If prompted, authenticate with Face ID or enter your passcode to open Apple Wallet. If your iPhone has Touch ID, double-click the Home button.
On your iPhone, go to Settings > [your payment app], then turn on Tap to Pay on iPhone Screen Lock.
Open Settings. In the “Search settings” bar, type NFC. Ensure NFC is turned on. Ensure that you enable contactless payments for the linked card.
Restart your iPhone or iPad. Make sure that your screen is clean and free of any debris or water. Disconnect any Lightning or USB-C accessories. If your screen works after removing an accessory, try using a different outlet, cable, or charger.
To set up Tap to Pay on iPhone, enable it in the payment app of your choice. To accept a payment, open the app, enter the amount, and present your iPhone to your customer. To pay, the customer holds their card horizontally or places their device directly over the payment symbol.
Open your iPhone's Settings and tap the Accessibility option. Tap "Touch" and hit the Back Tap option at the very bottom. Now, set both the "Double Tap" and "Triple Tap“ options to the ”None“ position. After that, force close Settings and then re-enable Back Tap by heading to Settings > Accessibility >Touch > Back Tap.
Edit: Looking now at Accessibility -> Touch options -> Back Tap offers "Lock Screen" for either Double or Triple tap options.
Double-Click Side Button: (on an iPhone with Face ID) Your cards and passes appear on the screen when you double-click the side button. Double-Click Home Button: (on an iPhone with a Home button) Your cards and passes appear on the screen when you double-click the Home button.
With Express Mode, you can use compatible cards, passes, and keys in the Wallet app with just a tap. With Express Mode, you can use some of your cards, keys, and passes in Apple Wallet without waking or unlocking your device, or authenticating with Face ID, Touch ID, or your passcode.
Go to Settings > Face ID & Passcode or Touch ID & Passcode, then turn on Wallet (below Allow Access When Locked).