Your openvpn version is older then 2.4 which do not support AES-256-GCM, hence you should upgrade it via below steps in your 16.04 linux:

curl -s https://swupdate.openvpn.net/repos/repo-public.gpg | apt-key add -
echo "deb http://build.openvpn.net/debian/openvpn/stable xenial main" > /etc/apt/sources.list.d/openvpn-aptrepo.list
apt update
apt install -y openvpn

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