The Client Certificates tool requires WMI connectivity to deliver and install the certificate. The "Invalid namespace" error is caused because the Extension Pack WMI Providers have not been deployed successfully. Typically this is a result of some breakdown in the RPC/WMI connectivity process. Resolving the WMI connectivity issues will allow the certificate to be successfully distributed to those clients.
As an alternative to using WMI to deploy these certificates, you can also use Group Policy.
Please refer to our guide: Distributing the WSUS Publishers Self-Signed Certficate Using Group Policy for detailed instructions.