The type or namespace name 'type/namespace' could not be found (are you missing a using directive or an assembly reference?) list code example

Example 1: The type or namespace name 'IEnumerator' could not be found (are you missing a using directive or an assembly reference?)

using System.Collections;

Example 2: error CS0246: The type or namespace name 'Slider' could not be found (are you missing a using directive or an assembly reference?)

using UnityEngine.UI;