You cannot assign a license key to a vCenter Server system.
You try to assign a license key to a vCenter Server system, but the operation is unsuccessful and you receive an error message.
You might be unable to assign a license key to a vCenter Server system because of the following reasons.
The license edition does not match the currently configured resources and features on vCenter Server. For example, while in evaluation mode, you add the vCenter Server system to a Linked Mode group. Then you try to assign a Foundation or an Essentials license key to the vCenter Server system. The operation is unsuccessful because the Foundation and Essentials license editions do not support the Linked Mode feature.
You assign an incorrect license key.
Upgrade the license edition to match the currently configured features and resources on vCenter Server. For example, you need a Standard or an Enterprise license key to be able to license a vCenter Server system that is in Linked Mode.
Assign a correct license key. To license vCenter Server, you need a vCenter Server license key.