Mapping USB keyboard Fn key to Option key with DefaultKeyBinding.dict
Using Karabiner event, it seems like keystrokes on Fn keys are not detected by the Mac. Can I fix it?


You can’t remap the Fn key as it’s an internal keyboard modifier and not sent to the OS

See this answer from a similar question for full details.

Basically, if a scan code isn’t sent by the keyboard to be received by the OS (and the Fn code isn’t), nothing can be remapped.