If you are en existing user and you already have adsense working elsewhere – there is no reason it should not work here. Before the ads start appearing on your website created usign Google Sites – Google adsense sends you a verification for Google sites to be accessed by adsense. This step may not be necessary for those who are signing up from Google Sites itself. May be you check your email and look for the verification email sent by Google Adsense. Hope this will help!