Apple pay is a system whereby you can add your supported credit card or debit card to your Apple device. For the phone and watch, you can tap on supported machines to pay. For the Mac, you just need to select Apple Pay on any online purchases (not as widely available). Adding a card to your device is a separate process from your card on file for your Apple account. For the phone, just open the wallet app. You can add a card there. Payments are charged directly on your credit/debit card. I don't know if prepaid credit cards are supported but I suspect they're not. More on reddit.com

In Settings->Control Center you can choose to enable the icon for Wallet, then you'd be able to tap that icon after swiping downwards from the upper right corner to open Control Center. That's the only thing that comes to my mind at the prompt "...by sliding a certain part of the screen in a certain way". More on reddit.com

Hi, first troubleshooting steps are to ensure your on the latest version of iOS (currently iOS 18.0.1) and ensure that your able to add your cards to your iPhone.  If your having issues adding a card check this support article first otherwise more information is below.

 if your unable to add your cards one of three main reasons may be behind it, the Apple Pay sensor may be damaged, or the secure element of the iPhone may be damaged(for Apple Pay Sensor or secure element damage check info for service below), or the iPhone was blacklisted from Apple Pay services(uncommon but has happened in cases where a previous owner used the iPhone to commit fraud).

 If you can add cards to Apple wallet but can’t use them to pay first make sure that the Apple wallet UI appears when double click the power button and make sure that your default card is able to authenticate using Face ID. 

If the Apple wallet UI did not appear then you may be having a software related issue, the recommendation at this step would be to factory reset the device using iTunes/Apple Devices app(for windows) or Finder/Apple Configurator(for Mac). 

If the Apple Wallet UI did appear but you couldn’t authenticate then you may be experiencing issues with Face ID. The first recommendation would be to reset Face ID and do a new scan. If a new scan fails or you were never able to setup Face ID you may have a hardware issue with your device that would require service(more details below) 

If the Apple Wallet UI appears and authentication is working then you may be experiencing an issue with the antenna’s, while you may be able to get WiFi, Bluetooth, and cellular signals fine the part of the antenna array used for NFC & Apple Pay processes may have been damaged and requires repair(more details below)

(If device may be damaged and requires service) If all software remedies above have failed you are likely looking at a hardware defect. This can be confirmed using Apple Diagnostic tool. If you don’t want to go to an Apple service provider you can use customer mode with iPhones running iOS 18 and up, simply turn off the iPhone, hold the up and down volume buttons at the same time, then while holding down plug the iPhone into a computer, keep holding them until you see the Apple logo. You’ll want to run the diagnostics suite Mobile Resource Inspector, this will tell you if it can spot any immediate hardware issues.

If your looking for an Apple Service location you can easily find one on apples website or using the Apple support app. Locations include Apple Stores, most Best Buy geek squad locations, some micro centers, and even some local independent stores. The technicians will be able to do multiple tests and walk you through available options.

!!Note if you recently purchased this iPhone refurbished and it’s determined it requires a repair it is going to be recommended to check the return policy from where you bought it from as typically iPhone 11 hardware repairs relating to Apple Pay issues end up costing $400. 

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Pass2uWallet in App Store. Now that I went to find it again, I see that there others. It would be worth comparing them. More on reddit.com