'System.Array' does not contain a definition for 'Count' and no extension method 'Count' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?)

IEnumerable doesn't have Count:

To resolve this server side error you just add the following namespaces,

 using System.Linq;
 

at the top of your source and make sure you've got a reference to the System.Core assembly.