For the ServiceName parameter of this policy try passing in the parameter value of "Sysmon64". As perhaps case-sensitivity is an issue.
Relating to Guest Configuration policies, there are some prerequisites that may be applicable to your use case. You can see these prerequisities here https://aka.ms/gcpol
I checked this particular builtin policy and found this note in the description about about non-compliance:
Machines are non-compliant if result of the Windows PowerShell command Get-Service do not include the service name with matching status as specified by the policy parameter.
Hope this helps