This is really an OpenVPN/iOS question, but my understanding is that, as the message says, OpenVPN on iOS does not support "tap" connections. So if you want to connect an iOS client, you must use a "tun" connection.
Since you control the VPN server, you must reconfigure it to use a "tun" connection. How to do that is an OpenVPN configuration question, and you won't find much help here, so I suggest you get help from OpenVPN experts: