What does "The type or namespace name x could not be found" mean?

hi im am making an hack and slash game from the BurgZergArcade tutorial and im facing an error that said : The type or namespace name `Skill' could not be found. Are you missing a using directive or an assembly reference? plz help me and explain to me whats wrong so i can improve my skill :)

code:

 using UnityEngine;
 using System.Collections;
 using System;        //added to acces the enum class
 
 public class BaseCharacter : MonoBehaviour {
     private string _name;
     private int _level;
     private uint _freeExp;
     
     private Attribute[] _primaryAttribute;
     private Vital[] _vital;
     private Skill[] _skill;
     
     public void Awake () {
         _name = string.Empty;
         _level = 0;
         _freeExp = 0;
         
         _primaryAttribut = new Attribute[Enum.GetValues(typeof(AttributeName)).Length];
         _vital = new Vital [Enum.GetValues(typeof(VitalName)).Length];
         _skill = new Skill [Enum.GetValues(typeof(VitalName)).Length];
         
         SetupPrimaryAttribute();
         SetupVitals();
         SetupSkills();
     }
     
     
     public string Name {
         get{ return _name;}
         set{name = value;}
     }
     
     public int Levvel {
         get{return _level;}
         set{_level = value;}
     }
     
     public uint FreeExp{
         get{return _freeExp;}
         set{_freeExp = value;}
     }
     
     public void AddExp (uint exp){
      _freeExp += exp;
         
         CalculateLevel();
     }
     
     //take the avg of all the players skill and assign that as the player level
     public void CalculateLevel(){
         
     }
     
     private void SetupPrimaryAttribute() {
         for(int cnt = 0; cnt < _primaryAttribute.Length; cnt++){
             _primaryAttribute[cnt] = new Attribute();
     }
     }
     
     private void SetupVitals(){
         for(int cnt = 0; cnt < _vital.Length; cnt++){
             _vital[cnt] = new Vital();
     }
     }
     
     private void SetupSkills(){
         for(int cnt = 0; cnt < _skill.Length; cnt++){
             _skill[cnt] = new Skill();
     }
     }
         
     public Attribute GetPrimaryAttribute(int index){
         return _primaryAttribute[index];
         
     }
 
                 
     public Vital GetVital(int index){
         return _vital[index];
     }
 
     public Skill GetSkill(int index){
         return _skill[index];
     }
 
     private void SetupVitalMofifiers(){
         //healh
         GetVital((int)VitalName.Health).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Constitution), .5f));
         
         //energy mana
         GetVital((int)VitalName.Energy).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Constitution), 1));
         
         //mana
         GetVital((int)VitalName.Mana).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Willpower), 1));
         }
 
     private void SetupSkillModifiers(){
         //melee Offence
         GetSkill((int)SkillName,Melee_Offence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Might),33f));
         GetSkill((int)SkillName,Melee_Offence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Nimbleness),33f));
         //melee Defence
         GetSkill((int)SkillName,Melee_Defence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Speed),33f));
         GetSkill((int)SkillName,Melee_Defence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Constitution),33f));
         //magic Offence
         GetSkill((int)SkillName,Magic_Offence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Concentration),33f));
         GetSkill((int)SkillName,Magic_Offence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.WillPower),33f));
         //Magic Defence
         GetSkill((int)SkillName,Magic_Defence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Concentration),33f));
         GetSkill((int)SkillName,Magic_Defence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Willpower),33f));
         //Ranged Offence
         GetSkill((int)SkillName,Ranged_Offence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Concentration),33f));
         GetSkill((int)SkillName,Ranged_Offence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Speed),33f));
         //Ranged Defence
         GetSkill((int)SkillName,Ranged_Defence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Speed),33f));
         GetSkill((int)SkillName,Ranged_Defence).AddModifier(new ModifyingAttribute(GetPrimaryAttribute((int)AttributeName.Nimbleness),33f));
 
     }
     
     public void StatUpdate(){
         for(int cnt = 0; cnt < _vital.Length; cnt++)
             _vital[cnt].Update();
         for(int cnt = 0; cnt < _skill.Length; cnt++)
             _skill[cnt].Update();
     }
 }