unity – Type or namespace name ‘Collider’ could not be found

I asked Mail.RU how to make a script so that when an object is touched, the game will restart. They sent me this:

using UnityEngine.SceneManagement;

public class RestartGameOnTouch : MonoBehaviour 
{ 
    // Attach this script to an object that
    // should restart the game when touched.
    
    void OnTriggerEnter(Collider other) 
    { 
        // Check if the player has touched
        // the object with this script. 
        if (other.CompareTag("Player")) 
        { 
            // Restart the game.
            RestartGame(); 
        } 
    } 
 
    void RestartGame() 
    { 
        // Here you can add any logic that needs
        // to be executed before restarting the 
        // game, for example, resetting counters,
        // player position, etc.
 
        // For simplicity, we'll reload the scene:
        var scene = SceneManager.GetActiveScene();
        SceneManager.LoadScene(scene.buildIndex); 
    } 
}

Then, using other answers from Mail.RU, I changed the restart method to:

    void RestartGame()  
    {  
        var scene = SceneManager.GetActiveScene(1);
        SceneManager.LoadScene(scene.buildIndex);
    }  

I don’t know what the problem is, but I get this error message:

Assets\Scenes\TOUGT RESTART.cs(5,25): error CS0246: The type or namespace name ‘Collider’ could not be found (are you missing a using directive or an assembly reference?)

When I try to run my game, it says that first I need to fix all compiler errors.

What do I need to do?