Maybe it’s because your SIM card isn’t supported or iPhone 5 is just too old. I would try to activate first without iPhone 5. If that works then its probably a problem with your iPhone 5.
Hello. I found this solution from a site: You tap the top edge of your watch toward the NFC reader on the register/terminal you're using. (https://www.androidcentral.com/how-set-samsung-pay-your-samsung-galaxy-watch-4)
