Static Routes without Gateways
It would be really useful to be able to define static routes without providing a gateway, such as OpenVPN without pushed routes.
Route and gateway belongs to each other. Where will you routing a traffic to??
Probably to a device by specifying a destination IP. So that IP is your gateway which you have to define first on pfSense.
You cannot specify an OpenVPN device as gateway device; the ovpnc devices are not listed, only LAN, WAN, and other ethernet interfaces. Thus, gateway monitoring does not work and the route is not accepted.
OpenVPN routing should be configured in the OpenVPN settings. Use the "Remote Network/s" box to enter the networks you want to route over the respective VPN.
If you want to route traffic over a OpenVPN client, assign an interface to the client instance. Interfaces > Assignments.
At "Available network ports" select the client instance (e.g. ovpnc1), hit Add, open the settings of the new interface, check Enable and set a proper name. No further configuration to make here!
If you have done that pfSense also add a virtual gateway to the vpn connection which can be used in firewall rules for policy routing or also for gateway monitoring.
But do not add static route to a vpn gateway! That's not recommended. As mentioned above, that is to be done in the OpenVPN settings.