UI.Selectable.interactable.
using UnityEngine;
using System.Collections;
using UnityEngine.UI; // required when using UI elements in scripts
public class Example : MonoBehaviour
{
public Button startButton;
public bool playersReady;
void Update()
{
// checks if the players are ready and if the start button is useable
if (playersReady == true && startButton.interactable == false)
{
//allows the start button to be used
startButton.interactable = true;
}
}
}