Who can join Keep Rewarding?
One thing that is great about Keep Rewarding is that everyone can join, as it is open to all countries. You just have to be at least 13 years old.
The number of opportunities will, however, vary from country to country.
However, they do mention in their FAQ section that there might be a few countries they block at their sole discretion. If you try to join but are not allowed to do so, this is likely the reason.
So you can join almost no matter where you are from, but I think you should, first of all, check out the best sites in your country, and then you can consider Keep Rewarding if you still need more sites to join.
But in most countries, there will be sites that give overall better opportunities, in my opinion.