SOLVED: Error message ResourceTypeNotSupported "Unsupported resource type: 'Microsoft.PolicyInsights/policyStates/summarize'" using the Azure REST API.
To generate a state summary of a of an Azure policy assignment you can use the Azure PowerShell cmdlet Get-AzPolicyStateSummary. However when you call this using REST you get the error https://management.azure.com/Subscriptions/ {{subscriptionId}} /providers/Microsoft.Authorization/policyAssignments/SecurityCenterBuiltIn/providers/Microsoft.PolicyInsights/policyStates/latest/summarize?api-version=2019-10-01 { "error" : { "code" : "ResourceTypeNotSupported" , "message" : "Unsupported resource type: 'Microsoft.PolicyInsights/policyStates/summarize'." } } This is because the method must be called with the verb POST (not GET).