You can't access https://graph.windows.net end point in graph explorer, you can try to run this in postman - https://graph.windows.net/<intuneTenant>/servicePrincipalsByAppId/0000000a-0000-0000-c000-000000000000/serviceEndpoints?api-version=1.6
If you are trying to get the list of service principle, you can use call GET - https://graph.microsoft.com/v1.0/servicePrincipals?$count=true
for more info about Working with Intune in Microsoft Graph -https://learn.microsoft.com/en-us/graph/intune-concept-overview and https://learn.microsoft.com/en-us/graph/api/resources/intune-graph-overview?view=graph-rest-1.0&preserve-view=true
Hope this help, please let us know you have any query
Thanks