I made this but why do i only jump one time

after i jump once and land i cant jump again, why?

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

public class PlayerJump : MonoBehaviour {

	public bool onGround;
	private Rigidbody rb;

	// Use this for initialization
	void Start () 
	{
		onGround = true;
		rb = GetComponent<Rigidbody> ();
	}
	
	// Update is called once per frame
	void Update () 
	{
		if (onGround) 
		{
			if (Input.GetKey (KeyCode.Space)) 
			{
				rb.velocity = new Vector3 (0f, 5f, 0f);
				onGround = false;
			}
		}
	}
	void onCollisionEnter (Collision other)
	{
		if (other.gameObject.CompareTag ("Plane")) 
		{
			onGround = true;
		}
	}
}

the only thing you need to do is to change “onCollisionEnter” to “OnCollisionEnter”. that’s the built-in method, should be uppercased.
@CadenGS_