This issue might occur because of

[![enter image description here][1]][1]

 - You may have a system custom master page that some placeholders were deleted or its visibility set to false.
 - The app installation service timer job is disabled.

To trace the issue, you need to set back the current master page to the default master page (not customized) to can load your Apps properly.and in this case you needs to add the missing placeholders to your custome master page as I have mentioned at 
[Add an App Stuck on Loading in SharePoint 2013][2]


  [1]: https://i.sstatic.net/Y75ib.png
  [2]: https://blog.devoworx.net/2015/10/17/your-apps-stuck-loading-in-sharepoint-2013/