Can you try removing the host from the inventory and try to re-add it back .
You have singular ESXi host that is running your VMs together with the vCenter.
You want to update it with Vsphere Update Manager -> Pre-Check Remediation Report is first stage for that.
This error message is caused by the fact that the VUM cannot evacuate your host (like migrate VMs and vCenter somewhere else) in order update it.
You need to add second ESXi host to the vCenter and add them both to the DRS cluster.
This is expected when you have a standalone ESXi host which has vCSA running on it.
thank you for your reply.
is there any explaination or where can i read this?
You're trying to move the ladder you're standing on...
Please read this: Pre-Check Remediation Report: One of the virtual machines in the cluster runs the Update Manager instance that you currently use. If you enable DRS on the cluster, vMotion can migrate the VM where Update Manager runs to ensure that the remediation process of the hosts in the cluster is successful.
But this is only possible if you have two or more hosts... In a cluster...