If your vSphere clusters are not being discovered by the PMO gateway appliance, the most common reason for this is VMware DRS not being enabled for the clusters.

PMO requires that VMware DRS is enabled in order to work with a cluster.

After you enable DRS on the cluster, it will take upto 30 minutes for Platform9 to discover that cluster and report it as part of the gateway authorization wizard.

To trigger this immediately, please follow this Tutorial to re-authorize the gateway.