This week I tried to enable SNMP service to my ESX servers in order to add them to my new monitor system, so I went to my vCenter and tried to enable the SNMP service and for some reason get an error that probably restart was solving this issue, I couldn’ restart the ESX during the work day and was so enthusiastic to add my hosts to the monitor system, eventually I found good way to fix it.
You can fix by open SSH to your ESX and run the following commands:
esxcli system snmp set --communities public
esxcli system snmp set --enable true
Then go back to you vCenter and start SNMP service and you can see that it runs successfully.
Hope was useful for you.