Chosen Solution
I have replaced crack screen on my Apple Watch S4. Every function is ok except Apple Pay. No matter what, it kept prompting error in setting up Apple Pay, even I have resetted, restored as new watch, updated OS ver to 6.2.5 is it because the NFC module in the watch is paired with the old screen? Anyway, to reset such module?
Did you transfer the NFC module on the back of the display over to your new display?