did you try it from ESXi shell
esxcli system snmp set --communities YOUR_STRING
esxcli system snmp set --enable true
esxcli network firewall ruleset set --ruleset-id snmp --allowed-all true
esxcli network firewall ruleset set --ruleset-id snmp --enabled true
/etc/init.d/snmpd restart
or look at VMware KB Cannot start the snmpd service from VMware vCenter Server (2056832) | VMware KB