I came across this error on a newly created Windows Server 2012 R2 domain controller in a lab environment.

Launch cmd and run: setspn -L <hostname>
You will see no entries for WSMAN/<hostname> or WSMAN/<FQDN>

Run ADSIEDIT.MSC
Browse to Domain > DC=domainname > OU=Domain Controllers >CN=<HOSTNAME>
Right click on CN=<HOSTNAME>, where <HOSTNAME> is the name of the server throwing the error.
Click the Security Tab
Click Add / “Network Service”
Then check (X) to allow access to “Validated write to service principal name”
Click OK

Restart the Windows Remote Management Service
Re-Run: setspn -L <hostname>
Locate entries for WSMAN/<hostname> or WSMAN/<FQDN>

You can also manually create the WSMAN SPN’s using setspn.exe.