Help with system inputs for a combo game.

I want to build a game based on DDR, where user has to type in a combo and if done correctly and fast enough it will damage the enemy. If a error is made or combo done to slow player will take damage.

I am struggling to grasp how to setup the combo system and have it load a new combo after damage is dealt.

using System.Collections;

using System.Collections.Generic;

using UnityEngine;

public class ComboBreaker : MonoBehaviour {

 public GameObject Set1P;
 public GameObject Set2P;
 public GameObject Set3P;
 public GameObject Set4P;
 public GameObject Set5P;
 public GameObject Set6P;
 public GameObject Set1E;
 public GameObject Set2E;
 public GameObject Set3E;
 public GameObject Set4E;
 public GameObject Set5E;
 public GameObject Set6E;

 public int PlayerHP = 5;
 public int EnemyHP = 5;

 public KeyCode[] combo;
 public int currentIndex = 0;


 void Start()
 {
     //StartCoroutine(functionName());

     Set1P = GameObject.FindGameObjectWithTag("S1P");     // Get all Combo Sets s1p= Set One Player/ s1e = Set One enemy
     Set2P = GameObject.FindGameObjectWithTag("S2P");
     Set3P = GameObject.FindGameObjectWithTag("S3P");
     Set4P = GameObject.FindGameObjectWithTag("S4P");
     Set5P = GameObject.FindGameObjectWithTag("S5P");
     Set6P = GameObject.FindGameObjectWithTag("S6P");

     Set1E = GameObject.FindGameObjectWithTag("S1E");
     Set2E = GameObject.FindGameObjectWithTag("S2E");
     Set3E = GameObject.FindGameObjectWithTag("S3E");
     Set4E = GameObject.FindGameObjectWithTag("S4E");
     Set5E = GameObject.FindGameObjectWithTag("S5E");
     Set6E = GameObject.FindGameObjectWithTag("S6E");

 }

 void Update()
 {

     if(EnemyHP > 0 || PlayerHP > 0)
     {

         System.Random rand = new System.Random();//get random variable for set
         int set = rand.Next(1, 2); // random between 1 and 6, only using one to test with 

         if (set == 1)    // Random = 1, Show Set 1 and complete there will be 6
         {
             Set1P.SetActive(true); // Set Active both set 1's
             Set1E.SetActive(true);
             Set2E.SetActive(false);
             Set3E.SetActive(false);
             Set4E.SetActive(false);
             Set5E.SetActive(false);
             Set6E.SetActive(false);
             Set2P.SetActive(false);
             Set3P.SetActive(false);
             Set4P.SetActive(false);
             Set5P.SetActive(false);
             Set6P.SetActive(false);


             int AIspeed1 = rand.Next(1, 4);  // time between 1 and 3 sec/key
             int AIspeed2 = rand.Next(1, 4);
             int AIspeed3 = rand.Next(1, 4);
             int AIspeed4 = rand.Next(1, 4);
             float AIcomplete = AIspeed1 + AIspeed2 + AIspeed3 + AIspeed4; // Ai's time to complete combo player must complete faster

             AIcomplete -= Time.deltaTime;

             if (Input.GetKeyDown(KeyCode.DownArrow) && AIcomplete > 0) // if key correct go to next or take damage
              {
                  if (Input.GetKeyDown(KeyCode.LeftArrow) && AIcomplete > 0)
                  {
                      print("Left key pressed");
                      if (Input.GetKeyDown(KeyCode.UpArrow) && AIcomplete > 0)
                      {
                          print("up key pressed");
                          if (Input.GetKeyDown(KeyCode.RightArrow) && AIcomplete > 0)
                          {
                              print("right key pressed");
                              --EnemyHP;
                          }
                          else if (Input.GetKeyDown(KeyCode.LeftArrow) || Input.GetKeyDown(KeyCode.UpArrow) || Input.GetKey(KeyCode.DownArrow))
                          {
                              --PlayerHP;
                          }
                      }
                      else if (Input.GetKeyDown(KeyCode.LeftArrow) || Input.GetKeyDown(KeyCode.RightArrow) || Input.GetKeyDown(KeyCode.DownArrow))
                      {
                          --PlayerHP;
                      }
                  }
                  else if (Input.GetKeyDown(KeyCode.RightArrow) || Input.GetKeyDown(KeyCode.UpArrow) || Input.GetKeyDown(KeyCode.DownArrow))
                  {
                      --PlayerHP;
                  }
              }
              else if (Input.GetKeyDown(KeyCode.LeftArrow) || Input.GetKeyDown(KeyCode.UpArrow) || Input.GetKeyDown(KeyCode.RightArrow))
              {
                  --PlayerHP;
              }

             if (PlayerHP <=0 || EnemyHP <= 0)
             {
                 GameOver();
             }

             print("looped");
         }
     }
     

 }

 void GameOver()
 {
     Time.timeScale = 0;
     //set GameOver Active
 }
 

}