GoKawiilApple Pay cards randomly refusing to work is one of the most common complaints with the feature. You might see a message saying the service is unavailable or that your payment couldn't go through. Usually, this is because the card expired or was reissued, and Wallet hasn't caught up yet.
The fix to try is removing the card and adding it back. Open the Wallet app and select the card that's giving you trouble. Tap the three-dot button, select Card Details, then scroll down and choose Remove Card. Add it again by tapping the Plus (+) button on Wallet's main screen, followed by Debit or Credit Card. It's a simple trick that should fix most minor issues. Just be aware that re-adding might require you to confirm with your bank or card provider through its app or a phone call. If the card hasn't ever worked in the first place, make sure your bank supports Apple Pay.
Having the wrong settings on your iPhone can also cause problems with the Wallet app. Outdated software can cause various features to misbehave, so try updating your device by going to Settings > General > Software Update. iOS updates often add new features to Wallet, which is another reason to keep your device updated.
Also, features like Apple Pay and Apple Cash are limited to certain regions. There's an Apple Support page listing every supported region and bank, which is worth checking out if you're traveling or have recently moved to another country. To confirm your iPhone's region, open Settings and go to General > Language & Region. Make sure the Region is set properly.