Damage System Doesnt Work?

Well, when is the function/method TakeDamage called? Replace TakeDamage with Update so you can check for the input key press each frame.

 var Health : float = 100;
 var MaxHealth : int = 100;
 var HealthBarTexture: Texture2D;
 var SelfDamage : int = 10;
  
 function Update()
 {
     if(Input.GetKeyDown(KeyCode.A))
     {
         Health -= 10;
     }
 }
  
 function OnGUI()
 {
     if (Health <= 0)
     {
     Health = 0;
     }
  
     var HealthPercent = Health / MaxHealth;
  
     var left = 50;
     var top = 50;
     var MaxHealthBarWidth = 500;
     var HealthBarWidth = MaxHealthBarWidth * HealthPercent;
     var height = 50;
  
     //Health Bar Texture
     GUI.DrawTexture(Rect(left,top,HealthBarWidth,height), HealthBarTexture, ScaleMode.StretchToFill, true, 1.0);
 }

OR

 var Health : float = 100;
 var MaxHealth : int = 100;
 var HealthBarTexture: Texture2D;
 var SelfDamage : int = 10;
  
 function Update ()
 {
     if(Input.GetKeyDown(KeyCode.A))
     {
         TakeDamage();
     }
 }
 
 function TakeDamage ()
 {
     Health -= 10;
 }
  
 function OnGUI()
 {
     if (Health <= 0)
     {
     Health = 0;
     }
  
     var HealthPercent = Health / MaxHealth;
  
     var left = 50;
     var top = 50;
     var MaxHealthBarWidth = 500;
     var HealthBarWidth = MaxHealthBarWidth * HealthPercent;
     var height = 50;
  
     //Health Bar Texture
     GUI.DrawTexture(Rect(left,top,HealthBarWidth,height), HealthBarTexture, ScaleMode.StretchToFill, true, 1.0);
 }