i have no clue what is wrong with the script (help)

Questions & Answers
1

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

public class Healthtry : MonoBehaviour {
public int Health;

public Text HealthBar;
public gameObject DeathGUI;

void Death()
{
    if (Health <= 0)
    {

        DeathGUI.SetActive(true);
    }
}

void Start()
{
    Health = 10;
    HealthBar.text = Health.ToString();
    DeathGUI.SetActive(false);
}

void HealthTimer()
{
    Health--;
    HealthBar.text = Health.ToString();
}
void Damage()
{
    Health = -1;

}
void Update()
{
    if (Input.GetMouseButtonDown(0) && (hit.transform.gameObject.name == "Damage")) {

        Damage();
    }

    void Kill()
    {
        Xp();
        TimeXp();

    }

    void health()
    {
        if (hit.transform.gameObject.name == "HealthPack")
        {
            Health += 5;

        }

    }

    void TimeXp()
    {
        Float Time = 5;
        Time -= deltaTime;
    }
    void Xp()
    {
        if (Time <= 0)
        {
            Xp.SetActive(false);
        }
        else if ()
        {
            Xp.SetActive(true);

        }

    }
}
2

public Text HealthBar;
public gameObject DeathGUI;

// set Health and Time to be class scope variables
Health = 0;
Time = 0;

 void Start()
 {
     Health = 10;
     Time = 5;
     HealthBar.text = Health.ToString();
     DeathGUI.SetActive(false);
 }

 void Update()
 {
     if (Input.GetMouseButtonDown(0) && (hit.transform.gameObject.name == "Damage")) // Assuming you have a section of code somewhere that handles collision data and are saving it to a variable named hit?
     {
         Damage();
     }

    Time -= Time.deltaTime;    // This is the way to call deltaTime and subtract it from a variable;
 }

 //void HealthTimer(){Health--;HealthBar.text = Health.ToString();}  // Removed function since it wasn't being called in Update, meaning it wasn't doing anything

void Damage()
{
     Health -= 1;    // You were setting the HP to -1 every frame the game was playing by saying (Health = -1) and calling this function in Update

     HealthBar.text = Health.ToString();    // Added this to display health after taking damage

    // Call our modified isDeath Function to see whether or not that damage killed us
    isDeath();
}

 void Kill()
 {
     // See if we gain xp from this
     Xp();

     //TimeXp();    // Removed this function, read comment in function section
 }

 void health()
 {
     if (hit.transform.gameObject.name == "HealthPack")    // I assume there is more code to this script?? What I mean is that you aren't just assuming you can call collision data without setting it to a variable first?
     {
         Health += 5;
     }
 }

 //void TimeXp(){Float Time = 5;Time -= deltaTime;}    // Commented this function out as moving the variable to the class's scope and changing it in the update function seemed to be more appropriate

 void Xp()
 {
     if (Time <= 0)    // You were attempting to access a variable that didn't exist in the scope of this function by trying to test "Time", Will work now that Time is a class scope variable
     {
         Xp.SetActive(false);    // Again I'm assuming there is information left out as I would take it you are setting a gameobject or script to false? Otherwise you may as well just do nothing if time < 0 and call a function from that script if you're giving them xp
     }
     else    // Changed to else{} because you had it checking for IF NOTHING by doing "else if(){...}"
     {
         Xp.SetActive(true);    // Same as above
     }
 }

// Modified the function to logically make sense in the name, and to call the kill function if we died.
void isDeath()
 {
     if (Health <= 0)
     {
           DeathGUI.SetActive(true);
           Kill();
     }
 }
}

There is a lot that is/was wrong with this, take the time to read over the comments because it will definitely help.

Assuming this was a 1-1 copy and paste from your class file the syntax was all messed up, all your functions were in the update function which never would’ve worked. A lot of calls are/were being made to variables that weren’t part of that functions scope at that time.