Accessibility access to control your computer is required to use many features including selecting menu items, manipulating windows, pressing buttons, typed string triggers and the window switcher.
You will need to enable Accessibility permissions for both Keyboard Maestro and Keyboard Maestro Engine.
On modern macOS, you enable Accessibility permissions by opening System Preferences, going to the Security & Privacy preferences, and in the Privacy pane select Accessibility and ensure that both Keyboard Maestro and Keyboard Maestro Engine are ticked.
⚠️ Note that in Big Sur and later, as well as the issues described here, you will need to ensure that either Keyboard Maestro is not listed, or has Input Monitoring permission - Input Monitoring permissions can override accessibility permissions.
⚠️ Note that for Catalina continues the tradition started in Mojave of being difficult with the Accessibility permissions, and I have several reports of people having troubles enabling accessibility for Keyboard Maestro Engine. You should report this to Apple using the Feedback mechanism. Remove the Keyboard Maestro Engine entry from the accessibility permissions, then quit and relaunch the Keyboard Maestro Engine (in Keyboard Maestro, File ➤ Quit Engine, File ➤ Launch Engine) to re-add it, and then you should be able to enable it.
⚠️ Note that since Mojave it is fairly common for the accessibility checkboxes to be on but Keyboard Maestro still not have permission. Generally toggling the accessibility permissions checkboxes off and on resolves the issue. Alternatively, deleting the entries and letting the system re-create them usually works if toggling is not sufficient.
If Keyboard Maestro or Keyboard Maestro Engine are not listed in the Accessibility list, try restarting each of them. In Keyboard Maestro, choose File ➤ Quit Engine, File ➤ Launch Engine to restart the Keyboard Maestro Engine. Then quit and relaunch Keyboard Maestro. If that does not work, try Restarting and repeat the process. Failing that, you can drag the Keyboard Maestro.app in to the list, or control-click on the Keyboard Maestro.app, and select Show Package Contents, Contents, MacOS and then drag the Keyboard Maestro Engine.app in to the list.
If you continue to have problems with enabling Accessibility permissions, ensure you only have one copy of Keyboard Maestro on your Mac, that it is in the /Applications folder, that you moved it there using the Finder and restart and try again.
If you continue to have problems, you can use the tccutil reset Accessibility command to reset the Accessibility permissions.
If you continue to have problems, contact support.