LibreELEC master is using the linux kernel CEC framework instead of the old RPi firmware CEC support. Don't try to use libcec with RPi libs in parallel to that, this will interfere really badly.
The error 16 you saw is caused by kodi running in the background and using the CEC adapter. If you stop kodi ("systemctl stop kodi") you can use cec-client as usual.
so long,
Hias