Diagnose: The subscription is not registered to use namespace 'Microsoft.X'

SYMPTOM
The subscription is not registered to use namespace 'Microsoft.X'

CAUSE
New or rarely-used subscriptions have not registered the resource provider namespace the deployment needs. The namespace named in the error is the exact one to register.

CONFIRM IT
CONFIRM: `az provider show --namespace Microsoft.X --query registrationState` returns NotRegistered.

FIX
FIX: `az provider register --namespace Microsoft.X`, poll until `az provider show` reports Registered, then retry the deployment. Needs subscription-level permission; a resource-group-scoped service principal cannot do this.

VERIFY
VERIFY: registrationState is Registered and the original create command succeeds.