When I go up to a retail stores tap to pay machine, my tap to pay doesn't work.

Apple Pay - Apple

According to the "How to pay in stores" video linked on this page, you

• Double-click the side button,
• Authenticate with Face ID, and then
• Tap the phone

Perhaps you were omitting the first two steps?

3jn wrote:

I've done all these steps, authenticated my face. It says "Hold Near Reader" but when I put it near one nothing goes through, perhaps its a physical phone issue. Not sure what to ask service though

• Not all terminals support tap to pay.
• The ones that do require the phone to be placed in a specific area on the terminal.
• You can contact Apple directly and they can perform diagnostics on your device to determine if it is a Hardware Issue .Contact Apple Support - Apple Support

Did you try moving the phone around a bit? NFC communications is very local, so there might be a particular area on the back of the phone that needs to be positioned over the reader.

If the problem is happening at a gas station, sometimes those have pumps with "tap" readers that are disabled. If you tap your card, you're told to insert it into a slot, instead.

If you've tried everything, and nothing works, you could try contacting Apple Support.

Are you sure that merchant accepts Apple Pay? Not all of the merchants do, and not all of the readers take Apple Pay either. I have my Apple Card, my travel credit card, and my bank debit card, and all of them are working for me. I agree with Servant of Cats, some of the readers are very picky about where you hold the phone near. Sometimes it is the top, the bottom, or one of the corners. I would check with the merchant to make sure they take it. Does this happen at all places you try to use your wallet?

EDIT: Servant of Cats, I'm pretty sure that link is the one they need. I took a look at it after I responded. It explains how to add cards to your wallet, how to use it, and others.

[EDIT - The link I had posted here was about accepting Tap to Pay payments on iPhones, not about making them. So not as useful as I hoped it might be.]