Why not just use the ready-made OpenVPN server package?

apt update && apt -y install ca-certificates wget net-tools gnupg

wget -qO - https://as-repository.openvpn.net/as-repo-public.gpg | apt-key add -

echo "deb http://as-repository.openvpn.net/as/debian buster main">/etc/apt/sources.list.d/openvpn-as-repo.list

apt update && apt -y install openvpn-as