To be more precise, you can't do a survey while e-rewards' server thinks you are in another country. Connect through a proxy server in the desired country and it'll be fine.

This is one of the most annoying aspects of e-rewards. I have plenty of time to waste on their surveys during long layovers between overseas flights, but then their servers don't want to let you! I mean, isn't it obvious that people interested in something like e-rewards might be people who travel?!