Error in build when Xcode associated with outlet instead of action in Button

I just touched the MAC, so I will answer based on the image.

a. Why doesn't the build pass when it should go back to the previous state at Cmd+z at 4?

I tried it here, too.
To undo IBOutlet and IBAction changes, you could undo them by Cmd+z on the StoryBoard.
Activating the code pane and Cmd+z does not seem to affect the StoryBoard file.

Does it mean that it is not allowed to link with Button and outlet?

It's not a bad idea at all.
Rather, IBOutlet is a necessary step to match UI parts with code.

That's the solution, but I'll write it as a guess.
Does the XCode log say unrecognized selector sent to instance?

Let me explain about the image
* UIButton is defined as button in IBOutlet.
* The UIButton Action method defines aaa.

If you start the app in this state and press button, signal SIGABRT will occur.
The logs are as follows:

2015-09-12 10:49:59.041 16535 [1319:25886] ***Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[_6535.ViewController onButtonTapped:]:unrecognized selector sent to instance 0x65'fee0fee0fee0f

The reason for the error is that UIButton defines onButtonTapped:, but there is no onButtonTapped: in the ViewController class.

*As shown in the above image, IBAction definitions that do not exist must not remain

Press Ctrl and click on the affected UI part to view the connection list (black pop-up screen) and see if there are any unnecessary connections left.