I offer my apps for free, and Underground ask users to pay bitcoins to download it, is it a normal situation or what?
That is correct behavior and is by design. The message that translates as "Please check that your billing address and method of payment are in a supported country." indicates that your testers are trying to use Amazon Underground from a country that is not supported, OR their Amazon account that they used to sign in to the Underground client were created in unsupported countries, or have billing address set to an unsupported country.
Please note that Amazon Underground is supported only in the following 20 countries: Andorra , Austria , Belgium , France , Germany , Gibraltar , Guernsey , Holy See (Vatican City State) , Ireland , Isle of Man , Italy , Jersey , Liechtenstein , Luxembourg , Monaco , San Marino , Spain , Switzerland , United Kingdom , United States (including territories and possessions).
Thanks for posting! Amazon Underground does not ask users to pay bitcoins to download apps. Applications are free to download. If you are seeing something different, please login to your Dev Portal account and raise a Contact Us case with us specifying your app's name / ID, and we will investigate. Thanks!