I wanted my animation to play whenever my character shoots the gun but currently the animation runs only when it fires the first time and not the others. I have the impression that it will be something very simple in the code but I have been around here and still have not found a solution. Can someone give me a hint, please?

Here is the animation part in the code:

private Animator myAnimator;
 private bool isFire;
 private string FireAnimHash = "isFire";

 void Awake()
 
 spriteRend = GetComponent<SpriteRenderer>();
 

 void Start()
 
 myAnimator = GetComponent<Animator>();
 myAnimator.enabled =true;
 myAnimator.SetBool (FireAnimHash ,isFire);
 

 private void Update()
 {
 
 AimArmAtMouse();
 

 if (Input.GetButtonDown("Fire1"))
 
 isFire = true;
 myAnimator.SetBool (FireAnimHash ,isFire);
 

Actually, you use Triggers, not bools, when animating these types of animations, which become true and then false.
Copy the below script and change the variable type to trigger from bool in animator tab in unity.
It should work.

 private Animator myAnimator;
 private string FireAnimHash = "isFire";

 void Awake()
 
 spriteRend = GetComponent<SpriteRenderer>();
 

 void Start()
 
 myAnimator = GetComponent<Animator>();
 myAnimator.enabled = true;
 

 private void Update()
 
 AimArmAtMouse();

 if (Input.GetButtonDown("Fire1"))
 
 myAnimator.SetTrigger(FireAnimHash);
 
 

I don't know if your isFire is set to false after playing the shooting animation. If it has been true, and you have not transitioned it to other animations in the mecanim and did not set the animation to loop.

You either need to set isFire = false somewhere in your code, or you need to use Animator.SetTrigger.

From Unity's Documentation

//Attach this script to a GameObject with an Animator component attached.
//For this example, create parameters in the Animator and name them “Crouch” and
// “Jump”

//Apply these parameters to your transitions between states

//This script allows you to trigger an Animator parameter and reset the other that
// could possibly still be active. Press the up and down arrow keys to do this.

using UnityEngine;

public class Example : MonoBehaviour

 Animator m_Animator;

 void Start()
 
 //Get the Animator attached to the GameObject you are intending to animate.
 m_Animator = gameObject.GetComponent<Animator>();
 

 void Update()
 
 //Press the up arrow button to reset the trigger and set another one
 if (Input.GetKey(KeyCode.UpArrow))
 
 //Reset the "Crouch" trigger
 m_Animator.ResetTrigger("Crouch");

 //Send the message to the Animator to activate the trigger parameter named "Jump"
 m_Animator.SetTrigger("Jump");
 

 if (Input.GetKey(KeyCode.DownArrow))
 
 //Reset the "Jump" trigger
 m_Animator.ResetTrigger("Jump");

 //Send the message to the Animator to activate the trigger parameter named "Crouch"
 m_Animator.SetTrigger("Crouch");