Okay we have solved the problem and in order to help everyone I am going to write the solution here. So basically the problem was that the original "iPhoto Library" on my hard drive appeared to be migrated that's why I couldn't migrate again.
To solve it,
Go and right click on "iPhoto Library," than go to the "get info."
You will see a similar screen to that;
From the name, delete the "migrated" part. When you open double click it you will be able to migrate it to "Photos" application again.
Thanks to everybody who helped.