“Hold near scanner” prompt isn’t coming up in Apple Pay on my Apple Watch

How to use Apple Pay on Apple Watch: Making payments

https://www.tomsguide.com/how-to/how-to-use-apple-pay-on-apple-watch

You double-pressed the flat oval side button, then selected the credit card, then held the face of the Apple Watch over the reader?

Getting the double-press and release timing just right can be an issue. You can make it more reliable by changing:

Apple Watch -> Settings -> Accessibility -> Side Button Click Speed

After changing this I never accidentally get the Emergency Services display, and Apple Pay always comes up.

“Side Button Click Speed” can adjust the press and release timing, so you get the Control Center display every time. Adjusting The “Side Button Click Speed” is also useful for Apple Pay’s double press and release. There is “Default” (sub second press and release timing needed), “Slow” (up to a second between press and release), and “Slowest” (up to 2 seconds between press and release). I like the “Slow” setting. And as I said, I do not accidentally get the wrong display.

NOTE: Slow and Slowest settings do not mean you have to wait a full second or 2 seconds between press and release. It just means you do not need to be super fast.

Did you activate the card on your watch? I know adding a card on you iPhone does not necessarily add it to your Apple Watch.