(fn) key is stuck on 'show desktop' command

If all else fails try the following: 1: Reset NVRAM or PRAM on your Mac - Apple Support; 2 - boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Try System Preferences > Keyboard > Modifier Keys... > Restore Defaults OR just change the Function key setting.