After upgrading WebsitePanel to WebsitePanel 2.0, your Web Application Gallery may not work and you may receive the following error:
Application Gallery module is unavailable:
To fix this problem, install the latest version of Microsoft Web Deploy 3.0. After installing this module, do not forget to restart IIS service.
If you are using the older version of WebsitePanel and you are receiving this error, kindly refer the following article: