We are sorry to hear that you have this issue.
The article “How to use GroupDocs.Annotation for .NET in an ASP.NET MVC 5 Project” is slightly obsolete and tells how to use some older version of GroupDocs.Annotation. As we understand, you want to create a test application and see, how it is working. So in order to do this and to eliminate such possible issues with old versions, we suggest you to download the latest package “GroupDocs.Annotation C# Demos”. It contains several projects, including “Groupdocs.Demo.Annotation.Mvc”, which contains all you need. After installing this package you will obtain a ready-to-use project, which uses the latest version of GroupDocs.Annotation.
If this exception still be present while using “Groupdocs.Demo.Annotation.Mvc” project, please give us more details about it, including stack trace and a line of code, where it occurs.