K780 doesn't have a hard switch to lock the function keys. Logitech provides a utility to do this on Windows and iOS but not on Linux. You need to manually remap the keys.
Below works for Arch Linux, other systemd based distros should be about the same.
There's a problem with the F1-F3 keys as they're hardware specific and don't emit an event if pressed on their own and therefore can't be remapped. I might be wrong as I haven't spend any time on researching that.