How to use ConfigurationManager

I want to use App.config for storing some setting.I tried to use the next code for getting a parameter from a config file.

private string GetSettingValue(string paramName){    return String.Format(ConfigurationManager.AppSettings[paramName]);}

I also added System.Configuration for it (I used a separate class), and in App.config file I have:

<?xml version="1.0" encoding="utf-8" ?><configuration><startup> <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" /></startup><appSettings><add key ="key1" value ="Sample" /></appSettings></configuration>

But I got an error while trying to use ConfigurationManager - ConfigurationManager can't exist in such context, but I already added System.Configuration. Or did I miss something?

EDIT:

class with config (full view)

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;using System.Configuration;namespace browser{    class ConfigFile    {        private string GetSettingValue(string paramName)        {            return String.Format(ConfigurationManager.AppSettings[paramName]);        }    }}

EDIT2

Add how it looks

This means the problem is not during using ConfigurationManger but before - the program "says" that it "doesn't know such element" as I understand the error - the "Element ConfigurationManager" doesn't exist in such context"

EDIT3

EDIT 4