Please ensure that the Register directive uses a fully qualified name and that you have specified the exact version of UI for ASP.NET AJAX that is installed on your machine. You can check the following help articles for information about including the Telerik controls in pages and Web Parts for SharePoint:
- Incorporating RadControls for ASP.NET Ajax in SharePoint 2013 WebSites
- Create an AJAX-enabled SharePoint WebPart that uses RadControls
If you are still having difficulties using the Telerik controls in SharePoint I will need to examine the setup of your page with the Register directive and your web.config file in order to determine what could be causing the issue at hand.