As specified in the hosted payments documentation, please make sure that the hosted payments button is added to a web page, which is part of the domain/url provided during merchant signup.

For example, if your website url is http://myawesomestore.com. The hosted payments button cannot be added to a web page different from that domain (http://notmyawesomestore.com)
   
This restriction was added as an extra security measure, so the hosted payments button cannot be added to websites other than the merchant's approved/owned website's.