As I know, we can not create a subscription under existing subscription. Each subscription is different, unique and treated as individual.
Under subscriptions, we generally create resource groups and not subscription
See the diagram here - https://video2.skills-academy.com/en-us/azure/cloud-adoption-framework/decision-guides/subscriptions/#application-category-strategy
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Please don't forget to Accept Answer and Up-vote if the response helped -- Vaibhav