[C#] Can't Control Movement While Jumping

Long story short, I'm new to Unity. This is a basic 2D Platformer code. I was wondering why I can't move in the air and how to fix that issue. Thank you.

Here is my code:

 private float movementInputDirection;

 private int amountOfJumpsLeft;
 private int facingDirection = 1;

 private bool isFacingRight = true;
 private bool isWalking;
 private bool isGrounded;
 private bool isTouchingWall;
 private bool isWallSliding;
 private bool canJump;

 private Rigidbody2D rb;
 private Animator anim;

 public int amountOfJumps = 1;

 public float movementSpeed = 10.0f;
 public float jumpForce = 16.0f;
 public float groundCheckRadius;
 public float wallCheckDistance;
 public float wallSlideSpeed;
 public float movementForceInAir;
 public float airDragMultiplier = 0.95f;
 public float variableJumpHeightMultiplier = 0.5f;
 public float wallHopForce;
 public float wallJumpForce;

 public Vector2 wallHopDirection;
 public Vector2 wallJumpDirection;

 public Transform groundCheck;
 public Transform wallCheck;

 public LayerMask whatIsGround;

 // Start is called before the first frame update
 void Start()
 {
     rb = GetComponent<Rigidbody2D>();
     anim = GetComponent<Animator>();
     amountOfJumpsLeft = amountOfJumps;
     wallHopDirection.Normalize();
     wallJumpDirection.Normalize();
 }

 // Update is called once per frame
 void Update()
 {
     CheckInput();
     CheckMovementDirection();
     UpdateAnimations();
     CheckIfCanJump();
     CheckIfWallSliding();
 }

 private void FixedUpdate()
 {
     ApplyMovement();
     CheckSurroundings();
 }

 private void CheckIfWallSliding()
 {
     if (isTouchingWall && !isGrounded && rb.velocity.y < 0)
     {
         isWallSliding = true;
     }
     else
     {
         isWallSliding = false;
     }
 }

 private void CheckSurroundings()
 {
     isGrounded = Physics2D.OverlapCircle(groundCheck.position, groundCheckRadius, whatIsGround);

     isTouchingWall = Physics2D.Raycast(wallCheck.position, transform.right, wallCheckDistance, whatIsGround);
 }

 private void CheckIfCanJump()
 {
     if ((isGrounded && rb.velocity.y <= 0) || isWallSliding)
     {
         amountOfJumpsLeft = amountOfJumps;
     }

     if (amountOfJumpsLeft <= 0)
     {
         canJump = false;
     }
     else
     {
         canJump = true;
     }

 }

 private void CheckMovementDirection()
 {
     if (isFacingRight && movementInputDirection < 0)
     {
         Flip();
     }
     else if (!isFacingRight && movementInputDirection > 0)
     {
         Flip();
     }

     if (rb.velocity.x != 0)
     {
         isWalking = true;
     }
     else
     {
         isWalking = false;
     }
 }

 private void UpdateAnimations()
 {
     anim.SetBool("isWalking", isWalking);
     anim.SetBool("isGrounded", isGrounded);
     anim.SetFloat("yVelocity", rb.velocity.y);
     anim.SetBool("isWallSliding", isWallSliding);
 }

 private void CheckInput()
 {
     movementInputDirection = Input.GetAxisRaw("Horizontal");

     if (Input.GetButtonDown("Jump"))
     {
         Jump();
     }

     if (Input.GetButtonUp("Jump"))
     {
         rb.velocity = new Vector2(rb.velocity.x, rb.velocity.y * variableJumpHeightMultiplier);
     }

 }

 private void Jump()
 {
     if (canJump && !isWallSliding)
     {
         rb.velocity = new Vector2(rb.velocity.x, jumpForce);
         amountOfJumpsLeft--;
     }
     else if (isWallSliding && movementInputDirection == 0 && canJump) //Wall hop
     {
         isWallSliding = false;
         amountOfJumpsLeft--;
         Vector2 forceToAdd = new Vector2(wallHopForce * wallHopDirection.x * -facingDirection, wallHopForce * wallHopDirection.y);
         rb.AddForce(forceToAdd, ForceMode2D.Impulse);
     }
     else if ((isWallSliding || isTouchingWall) && movementInputDirection != 0 && canJump)
     {
         isWallSliding = false;
         amountOfJumpsLeft--;
         Vector2 forceToAdd = new Vector2(wallJumpForce * wallJumpDirection.x * movementInputDirection, wallJumpForce * wallJumpDirection.y);
         rb.AddForce(forceToAdd, ForceMode2D.Impulse);
     }
 }

 private void ApplyMovement()
 {

     if (isGrounded)
     {
         rb.velocity = new Vector2(movementSpeed * movementInputDirection, rb.velocity.y);
     }
     else if (!isGrounded && !isWallSliding && movementInputDirection != 0)
     {
         Vector2 forceToAdd = new Vector2(movementForceInAir * movementInputDirection, 0);
         rb.AddForce(forceToAdd);

         if (Mathf.Abs(rb.velocity.x) > movementSpeed)
         {
             rb.velocity = new Vector2(movementSpeed * movementInputDirection, rb.velocity.y);
         }
     }
     else if (!isGrounded && !isWallSliding && movementInputDirection == 0)
     {
         rb.velocity = new Vector2(rb.velocity.x * airDragMultiplier, rb.velocity.y);
     }

     if (isWallSliding)
     {
         if (rb.velocity.y < -wallSlideSpeed)
         {
             rb.velocity = new Vector2(rb.velocity.x, -wallSlideSpeed);
         }
     }
 }

 private void Flip()
 {
     if (!isWallSliding)
     {
         facingDirection *= -1;
         isFacingRight = !isFacingRight;
         transform.Rotate(0.0f, 180.0f, 0.0f);
     }
 }

 private void OnDrawGizmos()
 {
     Gizmos.DrawWireSphere(groundCheck.position, groundCheckRadius);

     Gizmos.DrawLine(wallCheck.position, new Vector3(wallCheck.position.x + wallCheckDistance, wallCheck.position.y, wallCheck.position.z));
 }