Why would my app not be able to "attach" an action?

I've changed my app class type from 开发者_如何学运维NSObject to NSApplication and now I get the following message when testing my app:

Could not connect the action buttonClicked: to target of class NSApplication

What does this mean and how can I fix it?

---

NSApplication does not respond to the buttonClicked: method and, therefore, there is no way for the target/action connection to be successfully made.