Try changing the ownership of the .plist file:
sudo chown root /Library/LaunchDaemons/myfile.plist
sudo chgrp wheel /Library/LaunchDaemons/myfile.plist
or more simply, change the user and group in one command:
sudo chown root:wheel /Library/LaunchDaemons/myfile.plist
It is also worth noting that these root LaunchDaemons
can’t be world writable, for security reasons:
sudo chmod o-w /Library/LaunchDaemons/*