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SNMP Error tells you, that cacti can't get System Information about the target via SNMP. As many readings are fetched via SNMP, some configuration on the target system is needed.
1) Configure the Target's SNMP agent (e.g. /etc/snmpd/snmpd.conf or the like). Setting of some community string (a kind of password) is required, see http://www.net-snmp.org/docs/FAQ.html
2) start the SNMP Agent
3) enter the community string into cacti's Device configuration for that device