The host cannot be admitted to the cluster's current 
Enhanced vMotion Compatibility mode. 
Powered-on or suspended virtual machines 
on the host may be using CPU features hidden by that mode.
Oh boi, not this again :/

This scenario assumes you have at least 2 ESXi hosts of which at least one is a member or EVC-enabled cluster and a datastore that is mounted to both of these hosts and VCSA is not on the ESXi host that is part of EVC cluster. Yes, this is real case.

To add ESXi host into cluster you need vCenter server, but with the EVC error presented above only cold migration will work.

First of, you need do storage vMotion to a shared datastore, that could be NFS, iSCSi, that does not matter at all as long as it’s mounted on both hosts. Easy, just do it.

Then log in to the ESXi where VCSA currently resides, shut it down and unregister the VM. Next, log in to the destination ESXi and register the VCSA. This is why you needed shared datastore. Voila, VCSA will now boot on EVC cluster.

Then go back to vCenter and add host to the cluster. It will likely be required that you shutdown remaining VMs on this host, if any.