While changing WinRM/Config/Service/AllowUnencrypted setting, error message "There are no more endpoints available from the endpoint mapper." is thrown.
This seems to affect Windows Server 2008 R2 servers that have the Windows Firewall service disabled.
This issue is described on this article from Microsoft.
Enable and start the Windows Firewall service. Then make the pertinent WinRM changes. Windows Firewall service can be disabled after the changes have been made.
If enabling the Windows Firewall service is not allowed or there's a risk that connectivity to the server is compromised by the Firewall upon enabling, this setting can be changed through the registry.
Quest Software does not provide support for problems that arise from improper modification of the registry. The Windows registry contains information critical to your computer and applications. Make sure you back up the registry before modifying it. For more information on the Windows Registry Editor and how to back up and restore it, refer to Microsoft Article ID 256986 “Description of the Microsoft Windows registry” at Microsoft Support.