If you're using Boot Camp to start up from Microsoft Windows, set Startup Disk preferences to start up from macOS instead. Then shut down or restart and try again.Some key combinations don't work when your Mac is using a firmware password.If you're using a keyboard made for a PC, such as a keyboard with a Windows logo, try a keyboard made for Mac. Or use your built-in keyboard or a wired keyboard. If you're using a wireless keyboard, plug it into your Mac, if possible.Some keyboards have a light that flashes briefly at startup, indicating that the keyboard is recognized and ready for use. You might need to wait a few seconds before pressing the keys, to give your Mac more time to recognize the keyboard as it starts up.

Then press and hold the key combination after pressing the power button to turn on your Mac. If you can't shut down, press and hold the power button for up to 10 seconds, until your Mac turns off. If you can't get a key combination to work when restarting your Mac, shut down your Mac first.Press and hold all keys in the combination together, not one at a time.

Mac os x utilities screen