Could not find any resources appropriate for the specified culture or the neutral culture. Make sure “AjaxControlToolkit.Properties.Resources.NET4.resources” was correctly embedded or linked into assembly “AjaxControlToolkit” at compile time, or that all the satellite assemblies required are loadable and fully signed.

Solution:
Add a ScriptManager on the top of the page.

<asp:ScriptManager ID=”ScriptManager1″ runat=”server”>asp:ScriptManager>