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 Mac OS X Yosemite (10.10 GM1 through 10.10.2) has a serious problem adding applications to the accessibility pane properly. This bug was largely resolved in Yosemite 10.10.3. So if you are having problems with accessibility under Yosemite, upgrade to 10.10.3 or later, and then launch Keyboard Maestro.
If you continue to have problems with enabling Accessibility permissions, ensure you only have one copy of Keyboard Maestro on your Mac and restart and try again.
If you continue to have problems, contact support.