telerik asp.net ajax error creating control
The Telerik.Web.Design.dll assembly is no longer added to the GAC by the automated installer and this causes the design-time error if you upgrade via the Upgrade Wizard and your project did not have an explicit reference to the Telerik.Web.Design.dll file in the project folder. You can read more in this section of the documentation: Error Creating Control Failed To Create Designer after upgrade to R1 2018.
You can download the hotfix archive from your Telerik.com account, so you can easily copy the Telerik.Web.Design.dll assembly to your target location.