An unexpected error has occurred when i click "Add an app"

Root Cause: Issue with Site collection version
Solution:
I did below steps to solve this..
Go to My Site Site collection

Click on My Content
Click on All Site Content
Click on Tool
Click Add an App
Close your browser
Now you will not get any error
http://microsoft-techies.blogspot.com/2014/06/my-first-sharepoint-online-app.html