I had some difficulties enabling SNMPD from the GUI on ESXi 6.5 and kept recieving the following error
Failed – An error occurred during host configuration
Quick search let me here.
Running the below steps listed at the above link allowed me to start SNMPD in the EXSi GUI without issue
esxcli system snmp set -r esxcli system snmp set -c YOUR_STRING esxcli system snmp set -p 161 esxcli system snmp set -L "City, State, Country" esxcli system snmp set -C noc@example.com esxcli system snmp set -e yes