I live in the UAE, the Samsung watch and my phone was purchased in UAE. But i am not able to install the Samsung pay app in my watch 4. It shows that not supported in your region where as my uncles Galaxy watch 2 and watch 5 doesn't show this. They too bought the device from UAE.
For assistance, please visit one of our authorized Service Centers to check it for you. You may refer to the link which will guide you to the nearest service center to your location https://www.samsung.com/ae/support/service-center/