When creating a new Azure B2C Tenant, we may see the following error:

If so, this means that our Azure Subscription is not registered to make use of the Microsoft.AzureActiveDirectory Namespace and so cannot create the Tenant against the Subscription.
We can resolve this error in two ways.
(1) Open the Azure Power Shell and run the following command: az provider register --namespace Microsoft.AzureActiveDirectory

This will register the Azure Active Directory Namespace to the current Subscription – and so allow the Subscription to create a new Active Directory Tenant.
(2) This can also be done through the Azure Portal in the following area: