How to use emission.enable correctly?

Here is a simplified version of your above script. But before you read this, please consider the following for future usage:

If you have any questions, please leave a comment below this answer, and I'd be happy to help you out. Otherwise, please mark the answer as correct by clicking the check mark below the thumbs-down button!

If you want to define a pre-defined variable (i.e. one that is defined outside of a function), do not declare it's type before you define it. For instance:

  public GameObject go;
     void Start(){
     go = GameObject.Find("GameObject");
     }

Instead of

 public GameObject go;
 void Start(){
 GameObject go = GameObject.Find("GameObject");
 }

If you define the type before the variable name, the compiler assumes you are wanting to name it as a temporary variable, which will only exist within that function. If you try to reference it anywhere else - you will get an error.

I hope this rewrite will help you, and perhaps sway you from your toilet humor ;)

 using UnityEngine;
 
 public class FartEffect : MonoBehaviour
     {
     public ParticleSystem.EmissionModule airFreshener;
     public float timer;
 
     void Update()
         {
         SprayAirFreshener();
         CheckInput();
         SubtractTime();
         }
 
     void CheckInput()
         {
         //If we click the mouse or strike the space bar, add 1/4 of a second to the timer
         if (Input.GetMouseButtonDown(0) || Input.GetKeyDown(KeyCode.Space)) timer += 0.25f;
         }
 
     void SprayAirFreshener()
         {
         //If the timer is greater than zero, enable the air freshener
         airFreshener.enabled = (timer > 0) ? true : false;
         }
 
     void SubtractTime()
         {
         //Subtract 1 second from the timer each second, and set it to zero if less than 1 second
         timer -= Time.deltaTime;
         timer = (timer <= 0) ? 0 : timer;
         }
     }