An ESXi host might disconnect from vCenter Server or all ESXi hosts might disconnect from vCenter Server at the same time.
An ESXi host disconnects from vCenter Server when the host evaluation period or license expires. All ESXi hosts disconnect from vCenter Server when the evaluation period or the license of vCenter Server expire. You receive a licensing-related error message both when a single host disconnects and when all hosts disconnect. You cannot add hosts to the vCenter Server inventory. The hosts and the virtual machines on the hosts continue to run.
The 60-day evaluation period of the host has expired or the host license has expired.
The 60-day evaluation period of vCenter Server is expired or the vCenter Server license is expired.
Assign a vSphere license to the ESXi host and try to reconnect it to vCenter Server.
Assign a vCenter Server license to the vCenter Server system.