NethServer Version: NS7
Since OpenVPN 2.4 (Well, 2.3.18 really) the single systemd unit file was split into 2 files. One for a server configuration, the other for a client.
It appears that the CentOS rpm maintainer is still including the “old” single systemd unit file, which is no longer shipped as part of the OpenVPN tarball. Once that ceases to be included, the NS OpenVPN support will break.