Input does not contain a definition for 'GetKeyDown'

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class RuchGracza : MonoBehaviour
{
   public float szybkoscPoruszania;
   public float wysokoscSkoku;

   public void update()
   {
       if(Input.GetkeyDown(KeyCode.Space))
       {
          GetComponent<Rigidbody2D>().velocity = new Vector2(0, wysokoscSkoku);
       }

       if(Input.GetkeyDown(KeyCode.D))
       {
            GetComponent<Rigidbody2D>().velocity = new Vector2(szybkoscPoruszania, GetComponent<Rigidbody2D>().velocity.y);
            transform.rotation = new Quaternion(0f, 0f, 0f, 0f);
       }
       else if(Input.GetkeyDown(KeyCode.A))

       {
            GetComponent<Rigidbody2D>().velocity = new Vector2(-szybkoscPoruszania, GetComponent<Rigidbody2D>().velocity.y);
            transform.rotation = new Quaternion(0f, 180f, 0f, 0f);
       }
   }
}

Next time please actually write some description and not just drop your code without proper syntax highlighting for review. I edited your question and put your code in a proper code block.

This error has almost always the same reason: You created or imported a custom class or type called “Input”. Therefore when you just refer to “Input” you will now reach out to your own class instead of the “UnityEngine.Input” class. I would highly recommend to rename your own class to something else. Having name collisions with Unity’s core classes is never a good idea.

If you want to keep your Input class, you probably should put it in an actual namespace and not the global namespace. That way you can select which Input class you’re using based on what namespaces you include in your file. If you don’t want or can’t do that either, your last option is to always use UnityEngine.Input instead of Input whenever you want to use Unity’s Input class. I would not recommend that.

Apart from that you named your method update. Unity won’t call this method since it only calls the method Update. C# is case sensitive. Method and class names in C# usually use PascalCasea / UpperCamelCase.

Your methods are not capitalised correctly.
That includes Update. Methods should be PascalCase.

You should also be getting autocomplete and error highlighting in your IDE, which would mean you can’t make these basic mistakes. Make sure you have configured yours.