Q:

Why interface builder not let me to add uiview to uitableview?

I have a table view. I want to add some features so when user click on a cell another view opens and show details about that cell. I am trying to follow this part of video to add uiview:
youtube tutorial
But when I drag and drop it myself, it returns back and is not adding to tableview. Here is my storyboard.

A:

I see your attached image and as per that i think you have use UITableViewController which has mainview is UITableView.
But the video you are trying to follow , in that they have take a UIViewController which has mainview is "UIView". in that they have have drag an drop UITableview. And then they have added a another UIView in "mainview which is "UIVIEW" " not UITableview.
If you want to learn how to sue UITableview in UIViewController, there here is the tutorial link :
http://www.appcoda.com/ios-programming-tutorial-create-a-simple-table-view-app/  
If you follow this steps then also you will be able to create the same tableview which you have currently developed. And then you try to add a view to MAINVIEW. it will allow you to add it.

