Currently, WhatsApp has activated Payments UPI only for those who enrolled for its beta testing program. For those who didn’t and still want to have it enabled for their smartphone, there is a different approach.
It works on invite system just like WhatsApp’s video invite program which happened 3 years back.
If you want to get it enabled for your device, you can check out this tutorial: Trick to enable WhatsApp UPI on Android and iOS
It hardly takes 1 minute to get it enabled. If you have doubt then feel free to ask. Regards :)