PlayerMovement error

Unity Services
,
1

I’m making a 2.5d game and the character can only go left and right. But I have an error, whenever I press the “D” key the Player goes back and gets stuck but if I release the “D” key the character goes back to the normal position. Here’s the code but there’s some errors like verticalVelocity here.

using System;
using UnityEngine;

public class PlayerMovement : MonoBehaviour
{
    private PlayerControls controls;
    private CharacterController characterController;

    public float walkSpeed;
    public Vector3 movementDirection;

    private Vector2 moveInput;
    private Vector2 aimInput;
    private void Awake()
    {
        controls = new PlayerControls();

        controls.Character.Movement.performed += context => moveInput = context.ReadValue<Vector2>();
        controls.Character.Movement.canceled += context => moveInput = Vector2.zero;

        controls.Character.Aim.performed += context => aimInput = context.ReadValue<Vector2>();
        controls.Character.Aim.canceled += context => aimInput = Vector2.zero;
    }
    private void Start()
    {
        characterController = GetComponent<CharacterController>();
    }

    private void Update()
    {
        ApplyMovement();
    }

    private void ApplyMovement()
    {
        movementDirection = new Vector3(moveInput.x, 0, moveInput.y);
        ApplyGravity();

        if (movementDirection.magnitude > 0)
        {
            characterController.Move(movementDirection * Time.deltaTime * walkSpeed);
        }
    }

    private void ApplyGravity()
    {
        if (characterController.isGrounded == false)
        {
            verticalVelocity = verticalVelocity - 9.81f * Time.deltaTime;
            movementDirection.y = verticalVelocity;
        }
        else
            verticalVelocity = -.5f;
    }

    private void OnEnable()
    {
        controls.Enable();
    }

    private void OnDisable()
    {
        controls.Disable();
    }
}

so can anyone help to fix these errors?