I faced with that error during installing 18c Grid software on checking list screen.

If you dont want to hit this warning error You can follow below steps:

On each node within the Oracle RAC Database cluster, stop and disable the avahi services run the following commands:

# systemctl stop avahi-dnsconfd
# systemctl stop avahi-daemon
Warning: Stopping avahi-daemon, but it can still be activated by:
avahi-daemon.socket

To keep the avahi services off persistently across reboots, on each node run the following:

# systemctl disable avahi-dnsconfd
# systemctl disable avahi-daemon
rm ‘/etc/systemd/system/dbus-org.freedesktop.Avahi.service’
rm ‘/etc/systemd/system/multi-user.target.wants/avahi-daemon.service’
rm ‘/etc/systemd/system/sockets.target.wants/avahi-daemon.socket’

One other option is, If you hit this error during installation click Fix/Repair button and run  /tmp/CVU_XX/runfixup.sh