Quiz Scoreboard

Hello, I want to make a script for score, at each question answered right the player gets a point but I can't figure how to make the script, so here it is:

 using UnityEngine;
 using System.Collections;
 
 public class Final : MonoBehaviour {
 
     public GUIStyle myStyle;
     public int Score=0;
 
     // Use this for initialization
     void Start () {
 
     }
     // Update is called once per frame
     void Update () {
         if(Application.ExternalCall("RightQ1"))
             Score+= 1;
         }
 
     void OnGUI(){
         if(Score > 0)
             GUI.Box(new Rect(0,0,Screen.width,Screen.height)," ",myStyle);
     }
 
     void Awake(){
         DontDestroyOnLoad(transform.gameObject);
     }
 }

And I get this error: Assets/Scripts/Final.cs(16,17): error CS0029: Cannot implicitly convert type void' to bool' What am I doing wrong?