Hi himanshuonscale,
The following error message indicates that your tenant has the Orders feature enabled. Currently, when Orders is enabled, Subscribe/Amend APIs cannot be used, "Post account" in the quick start is one of them. See Orders API Migration Guidance for more information.
"message": "Subscription api cannot be used when order is enabled."
For tutorials on the Orders API, please see the "Use the REST API" section in Orders Tutorials. Hopefully, this helps.
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