UniFi switches and gateways take their SNMP configuration from the UniFi Network controller, not from each device individually.
In the UniFi Network application, go to Settings > Services > SNMP. This is a controller-wide setting, not a per-device one.
Turning this on and setting a community string pushes the configuration out to every adopted switch, gateway and access point automatically.
Even though SNMP is enabled centrally, Linivo still polls each device's own management IP directly; add switches, gateways and any access points you want visibility into as separate devices.