The variable has not been assigned

Hello guys, can you help me. I am newbie in unity. I tried to create oncollisionenter. I tried to do thing that when the object collide with another object, it will appear the UI Image but when I put the UI image to the variable box, it shows that the variable has not been assigned etc. Below are my coding.

void OnCollisionEnter(Collision other)
{
	if (other.gameObject.tag == "Player")
	{
		Time.timeScale = 0f;
		delay = 5f;
		Question.SetActive(true);
	}
}

}

SetActive Doesn’t work for UI elements !
Try this code:
Note:Don’t forget to include Using UnityEngine.UI;

public Image Questions;
void OnCollisionEnter(Collision other)
 {
     if (other.gameObject.tag == "Player")
     {
         Time.timeScale = 0f;
         delay = 5f;
         Questions.enabled = true;
     }
 }

Thanks @prakyathd801 , but when I changed the coding as you told, it shows Object reference not set to an instance of an object. Am I missing something here? Hope you can help me

void OnCollisionEnter(Collision other)
{
	if (other.gameObject.tag == "Player")
	{
		Time.timeScale = 0f;
		delay = 5f;
		Questions.enabled = true;
	}
}

}