Why GUI Doesn't appears?

You are displaying the message only for the frame that the button is clicked. This can be fixed by adding a couple variables to help display a message for a short time. Also, you should show us the whole script otherwise we can't really help you. I can also say a few things of advice. 1: You should clean up the script a bit and make it easier to look at. 2: Make sure that helmValue and coins are actually working. Maybe by adding a Debug.Log line in the code to show if the values are actually less than eachother or if there even set just in case this doesn't fix the problem! 3: You don't have to put if(var == true), you can just put the var's name for the equivalent result 4: Set the value of coins when the GUI is supposed to pop up. Basically, wherever the buyClicked var is set to true. The following is my own go at the script. I'm not really fluent in C# though, haha, so sorry if there are a few mistakes! (I'm a Javascripter ^-^)

 GUIStyle runnerButton;
 GUIStyle runnerText;
 bool buyClicked = false;
 bool displayMessage = false;
 string message = "";
 float timer = 0;
 int coins;
 int helmValue = 100;
 
 void OnGUI ()
 {
      
      if(buyClicked)
      {
           if(displayMessage)
           {
                GUI.Label(new Rect(Screen.width/2 - 115, Screen.height/2 - 45, 375, 187), "" + message, runnerText);

                //The following line adds the time between each frame, each frame
                timer += Time.deltaTime;
                if(timer >= 3)
                {
                     //Once the time is greater than 3, we basically disable the message
                     timer = 0;
                     message = "";
                     displayMessage = false;
                }
           }

           //No need to draw the same box twice.
           GUI.Box(new Rect(0, 0, Screen.width, Screen.height), "");
           GUI.Label(new Rect(Screen.width/2 - 115, Screen.height/2 - 45, 250, 125), "Do You Want To Buy It?", runnerText);
 
           if (GUI.Button (new Rect (Screen.width/2 - 100,Screen.height/2,100,60), "Yes", runnerButton))
           {
                //Here, we can log the current value of coins and helmValue to see
                //if the Player's coins is actually less than the Helmet's Cost
                Debug.Log("Coins: " + coins + " | Helmet Cost: " + helmValue);
                if(coins < helmValue)
                {
                     message = "You Don't Have Enough Money";
                     displayMessage = true;
                }
      
                //Don't need to put 'else if' if there is only one other possibility
                else
                {
                     PlayerPrefs.SetInt("Total_Coins", coins - helmValue);
                     PlayerPrefs.SetInt(helmOK, 1);
                     renderer.enabled = false;
                     buyClicked = false;
                }
           }
      
           if (GUI.Button (new Rect (Screen.width/2 + 20,Screen.height/2,100,60), "No", runnerButton))
           {
                buyClicked = false;
           }
      }
 }
 
 void OnTriggerEnter (hit : Collider)
 {
      //Here I set a quick scenario for if the player comes in contact with a helmet at the store.
      //Take note that I set the coins value when buyClicked is being set to true
      //instead of when the button's clicked.
      if(hit.tag == "Player")
      {
           buyClicked = true;
           coins = PlayerPrefs.GetInt("Total_Coins");
      }
 }

Alright, I really hope I helped you mate. Feel free to post anymore problems!! ;)