Here're some solutions.Try it out.
1. Granting full rights to the "Temporary ASP.NET Files" folder for your impersonation identity.
2.In the appropriate Temporary ASP.Net Files directory (located at: C:\Windows\Microsoft.NET\Framework64\v4.0.21006\Temporary ASP.NET Files[projectName]), manually copied the AjaxControlToolkit.dll.
For more info : Could not load file or assembly 'AjaxControlToolkit'
Could not load file or assembly 'AjaxControlToolkit'
I hope this will help to you.