The Keyboard Maestro Engine is a background only application that enables all of Keyboard Maestro’s features. It responds to your Hot Key presses, watches the time, tracks applications and maintains your clipboard history, handles remote web requests and receiving clipboards, and, of course, executes your Macro Actions. It should be running at all times, so we recommend you enable the “Launch Engine at Login” preference in the Keyboard Maestro General preference pane.
The Keyboard Maestro Engine is normally launched at login (if you have enabled that preference) or whenever you launch the Keyboard Maestro editor, and then remains running until you logout unless you explicitly quit the Keyboard Maestro Engine.
So if it is not running that would typically mean either that you have explicitly quit it or that something has gone wrong with the Engine.
You can launch it by selecting File ➤ Launch Engine in Keyboard Maestro, and then verify that the Keyboard Maestro Engine is running, which you can do most easily by looking for the Keyboard Maestro icon in the status menu on the right of the menu bar.
The engine, once running, should remain running, so if you find otherwise, something is wrong.
If you continue to have problems, contact support.