easy UI text question text doesnt update

Alright, so let's see what's wrong here. As you know, the faulty line is :

 coinText = coinText.GetComponent<Text>();

coinText is already a Text, so there's no point in trying to get its Text component. I guess what you meant to do was to get a Text attached to the same GameObject your script is :

 coinText = GetComponent<Text>();

What happens here is that when the script hits line 16, coinText has not been initialized, it is null. So you cannot call GetComponent on coinText, hence the NullRefException.

So you just have to remove coinText. and your script will work. However, using GetComponent in Update is a bad habit, so this script should be improved a bit. Put all your GetComponent calls in Awake : you only need to get references to your other Components once, not every frame.

 void Awake () 
 {
      // Always get your references in Awake or Start, not Update
      coinText = GetComponent<Text>();     
      coinScript = GetComponent<coincollision>();
 }
  
 void Update () 
 {
      // It's good practice to check if GetComponent found what you were looking for
      if (coinText != null && coinScript != null)
      {
           coinsCollected = coinScript.coinCollected;
           coinText.text = "Score: " + coinScript.coinCollected.ToString();
      }
  }

Another way of finding these components would be to Serialize them and set them in the Inspector via drag and drop.

 public class coinTextUpdate : MonoBehaviour 
 {
     [SerializeField]
     Text coinText;
     [SerializeField]
     coincollision coinScript;
 
     ...
 }

Then you will be able to see a Text field and a coincollision field in the Inspector. Simply drag and drop the GameObjects to which the Text and coincollision are attached (in this case, the same GameObject this script is attached to), and you're good to go, and you can even remove the GetComponent calls in Awake since you won't need them anymore.