SCCM Cloud Management Gateway Error: Task [CreateDeployment for service (CMG NAME)] has failed

I recently encountered this error while standing up a ConfigMgr CMG for a client, the error isn't obvious until you delve a little deeper into the log files. I'd instructed SCCM to create a new Azure resource group for the CMG, but it kept failing.

CloudMgr.log
ERROR: Resource Manager - Failed to finish deployment. Check [Monitor/Activity log] on Azure Portal for more information

ERROR: Resource Manager - Deployment operation details: {"value":[{"id":"/subscriptions/e672f87f-12ed-4c34-879b-8181e3f0e994/resourceGroups/####/providers/Microsoft.Resources/deployments/CreateCloudServiceb929f43b-abac-412b-aa3f-2920883c39d3/operations/F5DF9DE74E0F058F","operationId":"F5DF9DE74E0F058F","properties":{"provisioningOperation":"Create","provisioningState":"Failed","timestamp":"2019-01-22T08:05:33.5763846Z","duration":"PT3.789856S","trackingId":"e2a743a1-e659-44d5-bce8-22baf66e4ed4","statusCode":"Conflict","statusMessage":{"error":{"code":"MissingSubscriptionRegistration","message":"The subscription is not registered to use namespace 'Microsoft.ClassicCompute'. See https://aka.ms/rps-not-found for how to register subscriptions.","details":[{"code":"MissingSubscriptionRegistration","target":"Microsoft.ClassicCompute","message":"The subscription is not registered to use namespace 'Microsoft.ClassicCompute'. See https://aka.ms/rps-not-found for how to register subscriptions."}]}},"targetResource":{"id":"/subscriptions/e672f87f-12ed-4c34-879b-8181e3f0e994/resourceGroups/####/providers/Microsoft.ClassicCompute/domainNames/####","resourceType":"Microsoft.ClassicCompute/domainNames","resourceName":"####"}}}]}

ERROR: Exception occured for service ###### : Hyak.Common.CloudException: Failed to finish deployment~~   at Microsoft.ConfigurationManager.AzureManagement.ResourceManager.StartAndMonitorDeployment(String resourceGroupName, String deploymentName, Deployment deploymentProp, Int32 secondsToWait, Int32 timeoutInMinutes)~~   at Microsoft.ConfigurationManager.AzureManagement.ResourceManager.CreateCloudService(String resourceGroupName, String cloudServiceName, String location, Int32 timeoutInMinutes)~~   at Microsoft.ConfigurationManager.CloudServicesManager.CreateDeploymentTask.Start(Object taskState).

The key piece of information is hidden in the middle and reads: The subscription is not registered to use namespace 'Microsoft.ClassicCompute'

This quite clearly states the problem, I obviously need to register the resource provider 'Microsoft.ClassicCompute' on the Azure subscription being used to provision the CMG.

The fix

To register a resource provider on the subscription, follow these steps:

1. In the Azure portal, All Services > Subscriptions 

2. Select the subscription being used

3. Click Resource Providers

4. Find Microsoft.ClassicCompute in the list of available resource providers and hit Register

After a minute or two you'll see the resource provider registered with the subscription (you may need to hit refresh).

After registration the cloud management gateway installed successfully.