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.

I am working on Asp .Net 4.0 and found error while try to use Modal Popup Extendar:

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.

After some googling I found this Solution and works perfectly, I was forgot to add script manager to the page:

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Best quality Asp .Net Ajax Control Toolkit tutorials.