Troubleshooting Tip: Troubleshooting SNMP v3 or v1 on Cisco Switches

Enable the below debug commands on the Cisco switch:

#debug  snmp energywise

#debug  snmp headers

#debug  snmp requests

#debug  snmp packets

#terminal monitor

Then go to Network >> Inventory >> select the device you need and click on the Credentials tab, then click Validate Credentials.
You will see the following output on your Cisco switch CLI.

To disable debugging please type:
#undebug all

Useful link: How to troubleshoot SNMP communication issues from FNAC side:

https://community.fortinet.com/t5/FortiNAC/Technical-Note-Troubleshooting-SNMP-communication-issues/ta-p/198137?externalID=FD42045