X_NOT_IMPLEMENTED
Means
this driver does not implement the requested feature
Typical cause
an interface-complete driver whose remote half is unwritten
Fix
use the default driver, or implement the named method
Means
a planned command, or a driver whose remote half is unwritten
Typical cause
one of the commands in CLI reference's planned table
Fix
the fix names the closest shipped command — never "not a command", which would send you looking for a typo