This is likely an issue caused by a region lock on your phone. We offer extremely limited support of international models here in the U.S. Samsung Community. With that being said, to get Samsung Pay working, you'd likely have to enable the right CSC profile for your region, or flash the correct firmware onto the device for your respective country. Be very careful if you decide to do this option, because if you make a mistake while flashing firmware you can permanently brick your device.
This is an element of Samsung Knox, or possibly CSC, and seems like a security feature. Also, every country has their own unique rules and restrictions with banks, electronic communications, etc. If anyone would be able to answer this question definitively, it would be Samsung support. You can search online about flashing the correction regional firmware, or enabling the right CSC settings; you might find something I haven't thought of. The XDA forums might be able to help as well.
Every country has it's own rules and restrictions, sure. But, Apple Pay and Google Pay are able to switch regions in less than a minute. Do these rules and restrictions apply uniquely to Samsung Pay?