title: Drupal 8, Some module's checkbox are disabled.
body: <p>I'm a newbie in Drupal 8. I have a question about extends/modules.why are some modules checkbox disabled and I can't check themes.
  <a href="https://i.stack.imgur.com/7AWgr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7AWgr.png" alt="enter image description here"></a> </p>
  
  <p>As you could see, I can't check Mega Slider !!</p>