Hiding "Next level" button if not finished

Using the new 4.6 UI?

If so add a Canvas Group to it and you can alter the Alpha from 1 to 0 to hide the button, also Unclick BlockRaycast and Interactible so the button doesn't block mouse clicks in your game.

You can also gently reduce the alpha of a canvas group to fade out the button.

If the button is the only thing on the canvas then you should really base the canvasGroup on the actual canvas but you can also fade in and out individual elements by putting a canvas group on them.

EDIT:

Hold a reference to the button so:

 using UnityEngine.UI;
 
 ...
 
 public Button FinishButton;
 
 // then set the various stuff on the canvas group
 
 FinishButton.GetComponent<CanvasGroup>().alpha = 0; // or 1 to show
 FinishButton.GetComponent<CanvasGroup>().interactable(false);

Sorry for any typos as I don't have access to unity, if you just want a quick method you can just set the button's active to false ...

 FinishButton.SetActive(false);

Which is quicker but you can't do fancy fade in or out. As with the Alpha above being set to 1 to show it, set all the false values to true when you enable the button again.