OnMouseUp() not working like meant

I am building a tower defense, and I want to upgrade my turrets. I've been trying to use OnMouseUp and raycasting, but both have failed. When using OnMouseUp, it doesn't matter where I click, it will run the function anyways. And when using raycasting, the same thing happens, or nothing will happen at all.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Cannon : MonoBehaviour {
     public GameObject projectile;
     public float reloadTime = 1f;
     public float rotateSpeed = 5f;
     public float cooldown = .25f;
     public GameObject muzzleEffect;
     public float errorAmount = .001f;
     public Transform target;
     public Transform[] muzzlePositions;
     public Transform turretBall;
     public GameObject aimHere;
     public List<GameObject> targetList = new List<GameObject>();
     public float damage;
     public RaycastHit hit = new RaycastHit();
     public Ray ray;
     protected bool showUpgrades;
     
     private float nextFireTime;
     private float nextMoveTime;
     private Quaternion desiredRotation;
     private float aimError;
 
     void Start() {
         damage = 12.5f;
         showUpgrades = false;
     }
 
     void Update (){
         if(target) {
             if(Time.time >= nextMoveTime) {
                 CalculateAimPosition(target.position);
                 turretBall.rotation = Quaternion.Lerp(turretBall.rotation, desiredRotation, Time.deltaTime * rotateSpeed);
             }
             
             if(Time.time >= nextFireTime) {
                 FireProjectile();
             }
 
         }
 
         if(target == null && targetList.Count >= 1) {
             target = targetList[0].transform;
             aimHere = target.gameObject;
         }
     }
     
     void  OnTriggerEnter (Collider other){
         if(other.gameObject.tag == "AimPoint") {
             nextFireTime = Time.time + (reloadTime * .5f);
 //            target = other.gameObject.transform;
             targetList.Add(other.gameObject);
         }
     }
     
     void  OnTriggerExit (Collider other){
         if(other.gameObject.tag == "AimPoint") {
             targetList.Remove(other.gameObject);
         }
     }
     
     void  CalculateAimPosition (Vector3 targetPos){
         //Vector3 aimPoint = new Vector3(targetPos.x + aimError, targetPos.y + aimError, targetPos.z + aimError);
         Vector3 aimPoint = new Vector3(aimHere.transform.position.x, aimHere.transform.position.y, aimHere.transform.position.z);
         if (aimPoint != turretBall.position) {
             desiredRotation = Quaternion.LookRotation(turretBall.position - aimPoint);
         }
     }
     
     void  CalculateAimError (){
         aimError = Random.Range(-errorAmount, errorAmount);
     }
     
     void  FireProjectile (){
         //audio.Play();
         nextFireTime = Time.time + reloadTime;
         nextMoveTime = Time.time + cooldown;
         CalculateAimError();
         
         foreach(Transform theMuzzlePos in muzzlePositions) {
             Instantiate(projectile, theMuzzlePos.position, theMuzzlePos.rotation);
             //Instantiate(muzzleEffect, theMuzzlePos.position, theMuzzlePos.rotation);
         }
     }
 
     public void TargetIsDead() {
         Debug.Log("TargetIsDead");
         targetList.RemoveAt(0);
         FetchNextTarget();
     }
 
     void FetchNextTarget() {
         Debug.Log (targetList.Count);
     }
 
     void OnMouseUp() {
         if(showUpgrades) {
             showUpgrades = false;
         } else {
             showUpgrades = true;
         }
     }
 
     void OnGUI() {
         if(showUpgrades) {
             if(GUILayout.Button("Upgrade")) {
                 damage = damage + 10;
                 Debug.Log("Upgraded");
             }
         }
     }
 }