This message means that the OpenVPN was launched without specifying the needed parameter for the gateway, you need to include either one of the theese:
1)
--route network/IP [netmask] [gateway] [metric]
Add route to routing table after connection is established. Multiple routes can be specified. Routes will be automatically torn down in reverse order prior to TUN/TAP device close.
This option is intended as a convenience proxy for the route(8) shell command, while at the same time providing portable semantics across OpenVPN's platform space.
netmask default -- 255.255.255.255
gateway default -- taken from --route-gateway or the second parameter to --ifconfig when --dev tun is specified.
metric default -- taken from --route-metric otherwise 0.
The default can be specified by leaving an option blank or setting it to "default".
The network and gateway parameters can also be specified as a DNS or /etc/hosts file resolvable name, or as one of three special keywords:
vpn_gateway -- The remote VPN endpoint address (derived either from --route-gateway or the second parameter to --ifconfig when --dev tun is specified).
net_gateway -- The pre-existing IP default gateway, read from the routing table (not supported on all OSes).
remote_host -- The --remote address if OpenVPN is being run in client mode, and is undefined in server mode.
2)
--route-gateway gw|'dhcp'
Specify a default gateway gw for use with --route.
If dhcp is specified as the parameter, the gateway address will be extracted from a DHCP negotiation with the OpenVPN server-side LAN.
3)
--ifconfig l rn
Set TUN/TAP adapter parameters. l is the IP address of the local VPN endpoint. For TUN devices in point-to-point mode, rn is the IP address of the remote VPN endpoint. For TAP devices, or TUN devices used with --topology subnet, rn is the subnet mask of the virtual network segment which is being created or connected to.
For TUN devices, which facilitate virtual point-to-point IP connections (when used in --topology net30 or p2p mode), the proper usage of --ifconfig is to use two private IP addresses which are not a member of any existing subnet which is in use. The IP addresses may be consecutive and should have their order reversed on the remote peer. After the VPN is established, by pinging rn, you will be pinging across the VPN.
For TAP devices, which provide the ability to create virtual ethernet segments, or TUN devices in --topology subnet mode (which create virtual "multipoint networks"), --ifconfig is used to set an IP address and subnet mask just as a physical ethernet adapter would be similarly configured. If you are attempting to connect to a remote ethernet bridge, the IP address and subnet should be set to values which would be valid on the the bridged ethernet segment (note also that DHCP can be used for the same purpose).
This option, while primarily a proxy for the ifconfig(8) command, is designed to simplify TUN/TAP tunnel configuration by providing a standard interface to the different ifconfig implementations on different platforms.
--ifconfig parameters which are IP addresses can also be specified as a DNS or /etc/hosts file resolvable name.
For TAP devices, --ifconfig should not be used if the TAP interface will be getting an IP address lease from a DHCP server.
You can read here about the all of the configuration options:
https://community.openvpn.net/openvpn/wiki/Openvpn24ManPage
I suggest you to also take a look there at the
Example 3: A tunnel with full TLS-based security for the reference.
