Ensure that SNMP is enabled on all network devices with Read Only access and that SNMP is set to accept packets from the probe. Refer to your device documentation on how to setup SNMP for that device.
Some devices require that SNMP is turned on in multiple locations such as both Administration and the LAN interface.
The probe acts as a SYSLOG server. If you are planning to monitor SYSLOG, point the SYSLOG output to the probe.
Most devices and N-able N-central default to the "public" community string. If you change this value, you will need to be diligent in changing this string when running your discoveries. Change SNMP settings in N-able N-central by clicking Views > All Devices, clicking the device name and then clicking Settings > Monitoring Options tabs for each individual device.