Very strange error when building

I was working on a tutorial, and while I was working on it, running the build on Unity was going fine. When I wanted to export my build for WebGL, I started getting these errors that haven't occured before. When I switched back to the regular PC build settings, these errors persisted.

I have never used Rider and I don't know what it is (I think it's an editor but i dont know). My editor that I use is Notepad++. I haven't ever encountered these errors, while doing normal development. I uninstalled the Rider IDE and all the rider errors left but the Timeline one is still there.

This is the error that was cut off:

Library\PackageCache\com.unity.timeline@1.2.14\Editor\Window\TimelineWindow_Manipulators.cs(11,18): error CS0246: The type or namespace name 'RectangleZoom' could not be found (are you missing a using directive or an assembly reference?)

I googled the timeline error and it may have to do with camera controlling I did but I'm not sure what it is. Here is the script for the camera I use (i got most of it from a github thing but edited it somewhat for my purposes):

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 public class FlyCamera : MonoBehaviour {
     /*
     Writen by Windexglow 11-13-10.  Use it, edit it, steal it I don't care.
     Converted to C# 27-02-13 - no credit wanted.
     Simple flycam I made, since I couldn't find any others made public.
     Made simple to use (drag and drop, done) for regular keyboard layout
     wasd : basic movement
     shift : Makes camera accelerate
     space : Moves camera on X and Z axis only.  So camera doesn't gain any height*/
    
    
     public float mainSpeed = 10f; //regular speed
     public float shiftAdd = 10f; //multiplied by how long shift is held.  Basically running
     public float maxShift = 100f; //Maximum speed when holdin gshift
     public float camSens = 0.25f; //How sensitive it with mouse
     public bool is_active = false;
     public GameObject enable_button = null;
     private Vector3 lastMouse = new Vector3(255, 255, 255); //kind of in the middle of the screen, rather than at the top (play)
     private float totalRun= 1.0f;
    
    
     void Update () {
         if(is_active){
             lastMouse = Input.mousePosition - lastMouse ;
             lastMouse = new Vector3(-lastMouse.y * camSens, lastMouse.x * camSens, 0 );
             lastMouse = new Vector3(transform.eulerAngles.x + lastMouse.x , transform.eulerAngles.y + lastMouse.y, 0);
             transform.eulerAngles = lastMouse;
             lastMouse =  Input.mousePosition;
             //Mouse  camera angle done.
      
             //Keyboard commands
             //float f = 0.0f;
             Vector3 p = GetBaseInput();
             if (Input.GetKey (KeyCode.LeftShift)){
                 totalRun += Time.deltaTime;
                 p  = p * totalRun * shiftAdd;
                 p.x = Mathf.Clamp(p.x, -maxShift, maxShift);
                 p.y = Mathf.Clamp(p.y, -maxShift, maxShift);
                 p.z = Mathf.Clamp(p.z, -maxShift, maxShift);
             }else{
                 totalRun = Mathf.Clamp(totalRun * 0.5f, 1f, 1000f);
                 p = p * mainSpeed;
             }
      
             p = p * Time.deltaTime;
             Vector3 newPosition = transform.position;
             if (Input.GetKey(KeyCode.Space)){ //If player wants to move on X and Z axis only
                 transform.Translate(p);
                 newPosition.x = transform.position.x;
                 newPosition.z = transform.position.z;
                 transform.position = newPosition;
             }else{
                 transform.Translate(p);
             }
         }
        
         if(Input.GetKey(KeyCode.Escape)){
             is_active = false;
             enable_button.SetActive(true);
         }
         if(is_active && enable_button.activeSelf){
             lastMouse = Input.mousePosition;
             enable_button.SetActive(false);
         }
     }
    
     private Vector3 GetBaseInput() { //returns the basic values, if it's 0 than it's not active.
         Vector3 p_Velocity = new Vector3();
         if (Input.GetKey (KeyCode.W)){
             p_Velocity += new Vector3(0, 0 , 1);
         }
         if (Input.GetKey (KeyCode.S)){
             p_Velocity += new Vector3(0, 0, -1);
         }
         if (Input.GetKey (KeyCode.A)){
             p_Velocity += new Vector3(-1, 0, 0);
         }
         if (Input.GetKey (KeyCode.D)){
             p_Velocity += new Vector3(1, 0, 0);
         }
         return p_Velocity;
     }
    
 }