Static Routes on pfSense are managed at System > Routing, on the Routes tab. A route may be added to any defined gateway. Ensure the gateway is present before attempting to add a route.
Routes do not need to be added for networks which are directly connected to any interface of the firewall, and doing so may cause problems. Only define static routes for networks which must be reached via some other gateway, but cannot be reached via the default gateway.
Never add static routes for networks reachable via OpenVPN. Such routes are managed by OpenVPN itself using Remote Network definitions, not static routes.
When adding a static route, enter a Destination network and CIDR subnet mask, choose a Gateway by which this network can be reached, and enter a Description.
Only actual gateways, not gateway groups, may be chosen for a static route.