If you don’t live in one of those countries but still see this message, you will need to check all your device settings. Most likely, you have your country, region, regional format, or something of the sort set to a country that is on the list. Until that is changed, you’ll not be able to access Rewards.
Example of someone who had this issue is in screenshot below. You’ll see they had country for Belarus but regional format for Russian (Russia). This then told Brave they are from Russia, which at the time of this FAQ is not supported. Once they changed from Russian/Russia for their region and regional format, Rewards became available.