Hi
Apple Pay is available in selected countries:
iOS - Feature Availability - Apple
If you are within one of those regions, then check that your Apple Watch region is set correctly:
On your iPhone, in the Watch app, go to: My Watch (tab) > General > Language & Region > Region Format.
If they are already correct, then it may help to change and then reselect the same Region Format and also the same System Language.
Also check that you are following all of the steps here: