If you cannot apply a license to an asset, the license might not match the currently configured features and resources. When you assign a license to an asset, the license must be compatible with all the configured resources and features.

For example, suppose you add 10 ESX hosts to the vCenter Server inventory during the evaluation period. After the evaluation period expires, you try to assign a Foundation edition license to a vCenter Server system. The assignment operation fails because the Foundation edition allows a vCenter Server system to manage up to three hosts only. To correct this issue, you can upgrade the edition or you can remove seven hosts from the inventory.

As another example, suppose that you configure VMotion and DRS on a cluster of Enterprise edition hosts. Later, you try to assign Standard license keys to the hosts. This operation fails because the Standard edition does not include VMotion and DRS. You must assign Enterprise licenses to the ESX hosts or disable VMotion and DRS. For detailed information about how to disable features, see the VMware Knowledge Base.

Also, make sure you are applying the correct license key, as follows:

To license vCenter Server assets, you must apply a vCenter Server license key.

To license ESX/ESXi assets, you must apply a vSphere license key.