Mac OS X 10.12 Sierra made a change to a security feature that helps prevent you from opening potentially dangerous applications. However it does make it difficult to launch older applications or some apps that are just not signed by the developer for a variety of reasons.
You’ll get a message like “My App can’t be opened because the identity of the developer cannot be confirmed”.
In previous versions of Mac OS you had a security preference option to allow any application to be opened but this option no longer exists. Here’s what it looks like when this happens and what to do when you are trying to open an older app or one that is not signed.