"no server suitable for synchronization found" error when trying to run `ntpdate`

The problem was coming from network default restrictions given in /etc/ntp.conf. You may have to uncomment the following lines in ntp server configuration file:

# Clients from this (example!) subnet have unlimited access, but only if
# cryptographically authenticated.
#restrict 192.168.123.0 mask 255.255.255.0 notrust

to

# Clients from this (example!) subnet have unlimited access, but only if
# cryptographically authenticated.
restrict 192.168.123.0 mask 255.255.255.0

You may be being blocked from reaching ubuntu.pool.ntp.org

Try a different NTP server pool.

Ask your ISP (or Network Administrator) for a local NTP server (or cluster).

If you want to sync with external time ref you don't have to "listen".

Check if you can reach 0.ubuntu.pool.ntp.org (ping and nmap -p 123 -sU 0.ubuntu.pool.ntp.org)