Trying to use variables from 'FirstPersonController' to slow down player inside a box collider. Please help!

Hey guys, first of all thanks for reading and maybe helping ;-)

Im working on a script that slows a player while inside a box collider that is a trigger.

It works by dividing the speed variables from the 'FirstPersonController' Standard Unity asset by the 'SpeedDivide' variable inside my c# script.

When the player is inside the water, the script should play an audio sound (watersound) and decrease the Playerspeed by 50% (dividing by 2).

the script goes as follows: (its in c#)

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

public class SlowDownWater : MonoBehaviour {

public float SpeedDivide = 2; bool InWater = false; public AudioClip watersound;

 void Start()
 {
     GameObject MainCharacter = GameObject.Find("Player");
     var playerScript = MainCharacter.GetComponent<FirstPersonController>();
 }
 
  void OnTriggerEnter() 
  {
      
          InWater = true;
          playerScript.m_WalkSpeed /= SpeedDivide;
          playerScript.m_RunSpeed /= SpeedDivide;
          playerScript.m_JumpSpeed /= SpeedDivide;
          Debug.Log("LOG :MainCharacter in water");
      
  }
  
  void OnTriggerExit()
  {
          playerScript.m_WalkSpeed *= SpeedDivide;
          playerScript.m_RunSpeed *= SpeedDivide;
          playerScript.m_JumpSpeed *= SpeedDivide;
          InWater = false;
          Debug.Log("LOG: MainCharacter out of water");
  }

  void Update()
  {
     if(InWater == true) 
     {
         audio.PlayOneShot(watersound);
         Debug.Log("LOG : Sound has played!");        
         
     }
  }
  

}

now ive got 7 errors that all say something like:

Assets/Scripts/SlowDownWater.cs(29,5): error CS0103: The name playerScript' does not exist in the current context and 1 that says : Assets/Scripts/SlowDownWater.cs(21,49): error CS0246: The type or namespace name FirstPersonController' could not be found. Are you missing `UnityStandardAssets.Characters.FirstPerson' using directive?

I appreciate all help, keep in mind im still learning Unity and C# so there might be some basic bugs.