vCenter doesn't expose SNMP by default for most host and VM metrics; Linivo polls it through the vSphere API instead.
In vCenter's Administration > Users and Groups, create a dedicated account with the built-in Read-Only role rather than reusing an administrative login for polling.
The sensor needs to reach vCenter's API endpoint on 443; if the sensor sits in a different network segment, confirm nothing between the two blocks the connection before troubleshooting credentials.
Rather than adding hosts and VMs individually, point Linivo at vCenter itself; it discovers hosts, clusters and VMs automatically and keeps that inventory in sync as things are added, moved or decommissioned.