The version of the rad controls you have is different from the version in of Telerik.Web.UI referenced in your Sitefinity project. If you look a little below that error message there is a text that will tell you which version of the Telerik.Web.UI you need to reference.
To fix it check the Telerik.Web.UI version number in references in yourur project and it will run visual studio solution delete it and add new reference with the correct version. Then build your project and it should run without problems.
Write back if you encounter any more problems.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items