I'm getting the "missing a using directive or assembly reference" and no clue what's going wrong

I'm trying to allow a user to enter data into a textbox that will be added to the web.config file. I've added the relevent lines to the web.config file but when I make this class all goes wrong.

I keep getting the are you missing a using directive or assembly refenrence error whenever I try to run my app. I have looked at the other times this question has been asked and can't seem to figure out where I'm going wrong. The thing is that I am extremely new to Visual Studio and am just left blank at what could be the answer.

Below here is the class file that's generating the error. I hope I've included everything you need to assist me. Thank you.

 using System.Collections.Generic;
 using System.Linq;
 using System.Configuration; 


namespace WebConfigDemo
{    
public class CompanyConfigSection : ConfigurationSection   
{       
    [ConfigurationProperty("", IsRequired = true, IsDefaultCollection = true)]   
    public CompanyConfigCollection Companies       
    {
        get           
        {              
        return (CompanyConfigCollection)this[""];           
        }            
        set            
        {            
            this[""] = value;            
        }       
    }   
}     
public class CompanyConfigElement : ConfigurationElement   
{      
                [ConfigurationProperty("id", IsKey = true, IsRequired = true)]        
    public int Id        
                {            
                    get      
                {                
                        return (int)this["id"];            
                    }            
                    set           
                    {               
                        this["id"] = value;          
                }    
                }         
    [ConfigurationProperty("name", IsRequired = true)]        
    public string Name        
    {            
        get           
        {            
                    return this["nam