@Chandra, Suresh Welcome to Microsoft Q & A Community Forum. I understand that you are trying to assign policy initiative to your subscription. Below image provides the list of available regulatory compliances that are built in with azure.
I assume you are referring to this initiative NIST SP 800-53 Rev. You can just go to azure portal. Search for Policy service. Click on Definitions tab under Authoring Section. Choose Definition Type as "Initiative", Category as "Regulatory Compliance" and Search with "NIST". You should be able to see the built-in policies
Select the Initiative "NIST SP 800-53 Rev" and Assign it to subscription scope. On how to assign the policy, you can refer to this document. Let me know if you are looking for the same or if you have further questions.