You can toggle the meaning of the Fn key in the Keyboard system preferences.
You can set a hot key to any key (except modifiers, Caps Lock, the Fn key and some special keys like Eject or Power) together with any modifiers (Shift, Command, Control, Option - but not caps lock or Fn). You do so by adding a hot key trigger, and then clicking on it so it is lit up blue, and then pressing the key combination.
If the key is not registered when you press it, try just “A”. If this works, then the previous key combination was not valid for some reason (feel free to ask about that). Make sure you immediately clear the hot key, otherwise your “A” key will stop working and you’ll be very confused.
If you can’t set the hot key even to just an “A”, then either Secure Input Mode is on or Accessibility permissions are not enabled for Keyboard Maestro and Keyboard Maestro Engine.