I need a help to solve my in game bug!

[PriomDeb]

My player character are able to go from left screen edge to right and right to left. But it is not moving from top screen edge to bottom and bottom to top. What can I do now ?

[PriomDeb]

This is my script.

using UnityEngine;
using UnityEngine.InputSystem;

public class Movement : MonoBehaviour
{
[SerializeField] private float forceMagnitude;
[SerializeField] private float maxVelocity;
[SerializeField] private float rotationSpeed;
[SerializeField] private string isRunning = “isRunning”;
[SerializeField] private float adjustViewPortTransition = 0.2f;

private Rigidbody rb;
private Camera mainCamera;

private Animator animator;

private Vector3 movementDirection;

void Start()
{
    rb = GetComponent<Rigidbody>();
    mainCamera = Camera.main;

    animator = GetComponent<Animator>();
}

void Update()
{
    ProcessInput();

    KeepPlayerOnScreen();

    RotateToFaceVelocity();
}

// 2. Updating Movement.
void FixedUpdate()
{
    if(movementDirection == Vector3.zero) { return; }

    rb.AddForce(movementDirection * forceMagnitude * Time.deltaTime, ForceMode.Force);

    rb.velocity = Vector3.ClampMagnitude(rb.velocity, maxVelocity);
}

// 1. Processing Touch Input
void ProcessInput()
{
    if (Touchscreen.current.primaryTouch.press.isPressed)
    {
        animator.SetBool(isRunning, true);

        Vector2 touchPosition = Touchscreen.current.primaryTouch.position.ReadValue();

        Vector3 worldPosition = mainCamera.ScreenToWorldPoint(touchPosition);

        // Getting a Vector3 from PlayerObject to TouchPoint
        movementDirection = transform.position - worldPosition;
        movementDirection.z = 0;
        movementDirection.Normalize();
        
    } else
    {
        movementDirection = Vector3.zero;
    }
}

// 2. Processing Rotation
private void RotateToFaceVelocity()
{
    if(rb.velocity == Vector3.zero) { return; }

    Quaternion targetRotation = Quaternion.LookRotation(rb.velocity, Vector3.back);

    transform.rotation = Quaternion.Lerp(transform.rotation, targetRotation, rotationSpeed * Time.deltaTime);
}

private void KeepPlayerOnScreen()
{
    Vector3 newPosition = transform.position;
    Vector3 viewportPosition = mainCamera.WorldToViewportPoint(transform.position);

    if (viewportPosition.x > 1)
    {
        newPosition.x = -newPosition.x + adjustViewPortTransition;
    }

    else if (viewportPosition.x < 0)
    {
        newPosition.x = -newPosition.x - adjustViewPortTransition;
    }

    else if (viewportPosition.y > 1)
    {
        newPosition.y = -newPosition.y + adjustViewPortTransition;
    }

    else if (viewportPosition.y < 0)
    {
        newPosition.y = -newPosition.y - adjustViewPortTransition;
    }

    transform.position = newPosition;
}

}

[PriomDeb]

I solve the bug. The problem was with the camera position. The camera position of y value was 5. It should be 0. I set the y position of main camera to 0 and it was align straight center. Now my problem is solved.