Add ESXi host to a cluster on a domain from the SDDC manager option is grayed out.
This operation is not allowed because cluster is imported
Use vSphere to edit the cluster.
This message appears exclusively on brownfield imported environments
VCF 9.0
The domain that you are adding the cluster or host to is not yet on version 9.0
You have not imported the hosts using the brownfield import tool
All components in the workload domain need to match VCF BOM in order to manage the Workload Domain from VCF Operations and SDDC Manager
Ensure that the hosts have been imported using the information from this KB:
Add host manually from the vCenter and use Sync inventory option from VCF Operations or Sync Version API from SDDC Manager.
Complete any necessary config drift upgrades from VCF Operations > Lifecycle Management page.