PlayerController.cs

using UnityEngine;

[RequireComponent(typeof(Rigidbody2D))]
public class PlayerController : MonoBehaviour
{
    private const float WALK_SPEED = 2;
    private const float JUMP_FORCE = 250;
    private Vector2 _jumpVector;
    private bool _queueJump;
    private bool _queueWalkLeft;
    private bool _queueWalkRight;

    private Rigidbody2D _rigidbody;

    private void Start()
    {
        _rigidbody = GetComponent<Rigidbody2D>();
        _jumpVector = new Vector2(0, JUMP_FORCE);
    }

    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.UpArrow)) _queueJump = true;
        if (Input.GetKey(KeyCode.LeftArrow)) _queueWalkLeft = true;
        if (Input.GetKey(KeyCode.RightArrow)) _queueWalkRight = true;
    }

    private void FixedUpdate()
    {
        var velocity = _rigidbody.velocity;

        if (_queueJump) _rigidbody.AddForce(_jumpVector);

        if (_queueWalkLeft == _queueWalkRight)
        {
            velocity = new Vector2(0, _rigidbody.velocity.y);
        }
        else
        {
            if (_queueWalkRight) velocity = new Vector2(WALK_SPEED, _rigidbody.velocity.y);
            if (_queueWalkLeft) velocity = new Vector2(-WALK_SPEED, _rigidbody.velocity.y);
        }

        _rigidbody.velocity = velocity;

        _queueJump = false;
        _queueWalkLeft = false;
        _queueWalkRight = false;
    }
}