Boyum Add-on Issue - "Unexpected Error: System.IO.FileNotFoundException: Could not load file or assembly 'System.Core, Version=x.x.0.0 ...'"

This error means that the .NET Framework required for our add-on to run is not installed, or the installation is corrupted. In order to fix it, please do the following

• If you have the same .NET version already installed that is showing on the error, repair the .NET installation 
• Otherwise, install the Microsoft .Net Framework

If this happens on a Windows Server 2008, 2008R2

• In the Server Manager interface, select Add Features to display a list of possible features
• In the Select Features interface, expand .NET Framework 3.5.1 Features
• Once you expand .NET Framework 3.5.1 Features, you will see two checkboxes. One for .NET Framework 3.5.1 and the other for WCF Activation. Check the box next to.NET Framework 3.5.1 and click Next.
• In the Confirm Installation Selections interface, review the selections and then click Install.
• Allow the installation process to complete and then click Close.