Error: Input string was not in the correct format - HELP!

I am creating an android game hill climb racing style. but I have a problem with the progress saving and and car upgrading.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.SceneManagement;
 using UnityEngine.UI;
 
 public class MenuScript : MonoBehaviour {
 
     public GameObject levelChanger;
     public GameObject exitPanel;
     public GameObject garagePanel;
     public Image[] cars;
     public Text coinText;
     public Button[] bttns;
     public Text[] carsText;
     public string[] levels;
     private char del = 'f';
     public Button[] updateBttns;
     saveToJson stj = new saveToJson();
     private int carForT;
 
     public GameObject ScrollPanel;
     public GameObject JeepPanel;
     public GameObject SportCarPanel;
     public Button chooseJeepBttn;
     public Button chooseSportCarBttn;
     public GameObject JeepUpgradePanel;
     public GameObject SportCarUpgradePanel;
     public Button UpgradeJeepButton;
     public Button UpgradeSportCarButton;
 
 
 
 
 
 
     void Awake()
     {
         if (!PlayerPrefs.HasKey("sv"))
         {
             for (int i = 0; i < stj.t.Length; i++) stj.t[i] = "0f0f0";
             string json = JsonUtility.ToJson(stj);
             PlayerPrefs.SetString("sv", json);
         }
         else
         {
             stj = JsonUtility.FromJson<saveToJson>(PlayerPrefs.GetString("sv"));
             for (int i = 0; i < stj.lvls.Length; i++) if (stj.lvls[i] == 3) { bttns[i].interactable = true; carsText[i].gameObject.SetActive(false); }
             if (data.gameStarted) data.dat(stj.coins, stj.car, stj.t, stj.lvls);
         }
     }
 
 
     void Start()
     {
         cars[data.car].color = Color.white;
     }
 
     void Update()
     {
         if (garagePanel.activeSelf == true && Input.GetKeyDown(KeyCode.Escape))
         {
             garagePanel.SetActive(false);
         }
         else if (levelChanger.activeSelf == true && Input.GetKeyDown(KeyCode.Escape))
         {
             levelChanger.SetActive(false);
         }
         else if (exitPanel.activeSelf == false && Input.GetKeyDown(KeyCode.Escape))
         {
             exitPanel.SetActive(true);
         }
         else if (Input.GetKeyDown(KeyCode.Escape))
         {
             exitPanel.SetActive(false);
         }
         coinText.text = PlayerPrefs.GetInt("coins").ToString();
 
     }
         
     public void OnClickExitExit()
     {
         exitPanel.SetActive(false);
     }
     public void OnClickExitPanel()
     {
         exitPanel.SetActive(true);
     }
     public void OnClickExitGaragePanel()
     {
         garagePanel.SetActive(false);
     }
     public void OnClickExitScrollPanel()
     {
         ScrollPanel.SetActive(false);
     }
     public void OpenJeepUpgrade()
     {
         JeepUpgradePanel.SetActive(true);
     }
     public void OpenSportCarUpgrade()
     {
         SportCarUpgradePanel.SetActive (true);
     }
         public void closeJeepUpgrader(){
         JeepUpgradePanel.SetActive (false);
     }
     public void closeSportCarUpgrader(){
         SportCarUpgradePanel.SetActive (false);
     }
         
     public void OnClickStart()
     {
         levelChanger.SetActive(true);
     }
     public void OnClickExit()
     {
         Application.Quit();
     }
     public void levelBttns(int level)
     {
         data.gameStarted = false;
         SceneManager.LoadScene(level);
     }
     public void carChanger(int car)
     {
         data.car = car;
         for (int c = 0; c < cars.Length; c++)
         {
             cars[c].color = Color.white;
             cars[car].color = Color.white;
         }
     }
     public void carTuning(int carPart)
     {
         string[] carforT = data.t[carForT].Split(del);
         int updateState = int.Parse(carforT[carPart]) + 1;
         if (updateState <= 3 && 5 * updateState <= data.coins)
         {
             carforT[carPart] = updateState.ToString();
             data.t[carForT] = carforT[0] + "f" + carforT[1] + "f" + carforT[2];
             data.coins -= 5 * updateState;
         }
         if (int.Parse(carforT[carPart]) == 3 || stj.coins < 5 * updateState)
         {
             updateBttns[carPart].interactable = false;
         }
         print(data.t[carForT]);
     }
     public void carForTun(int car)
     {
         carForT = car;
     }
     void OnApplicationQuit()
     {
         stj.dat(data.coins,data.car,data.t,data.lvls);
         string json = JsonUtility.ToJson(stj);
         PlayerPrefs.SetString("sv", json);
     }
 }
 public class saveToJson
 {
     public int coins = 0,car = 0;
     public string[] t = new string[3];
     public int[] lvls = new int[2];
 
     public void dat(int coins, int car, string[] t, int[] lvls)
     {
         this.coins = coins;
         this.car = car;
         this.t = t;
         this.lvls = lvls;
     }
 }
 public static class data
 {
     public static int coins = 0, car = 0;
     public static string[] t = new string[3];
     public static int[] lvls = new int[2];
     public static bool gameStarted = true;
 
     public static void dat(int coins,int car, string[] t,int[] lvls)
     {
         data.coins = coins;
         data.car = car;
         data.t = t;
         data.lvls = lvls;
     }
 }