Hi,
On the server you need firewall rules on the tun interface/subnet in the forward chain using the client(s) tunnel IP.
If using --client-to-client in the server config, you need to remove it.
Example:
https://backreference.org/2010/05/02/controlling-client-to-client-connections-in-openvpn/
Probably helpful diagram:
https://community.openvpn.net/openvpn/wiki/HowPacketsFlow
.