That Get-KmsCluster is part of the VMware.VimAutomation.Storage module.
Like Kamen already said, you don't need to specify that module prefix.
The problem is that the Get-KmsCluster cmdlet is also part of the VMware.VMEncryption module.
Just try with Get-KmsCluster, without prefix.
Or try with VMware.VMEncryption\Get-KmsCluster