Hello Vipal Mehta,
Welcome to the Microsoft Q&A and thank you for posting your questions here.
I understand that you are having issues with role creation in Azure Cosmos DB for PostgreSQL.
Hope you are not performing actions that might conflict with the role creation, such as creating or deleting child resources (databases, containers, etc.) before the role synchronization completes. Ensure the creation of the role completed and synchronized, also, check via the Azure portal, the Azure CLI and Database to confirm the role exist. Then, check out these links for more detailed steps about role creation: https://video2.skills-academy.com/en-us/azure/cosmos-db/role-based-access-control and https://github.com/MicrosoftDocs/azure-docs/issues/114839.
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I hope this is helpful! Do not hesitate to let me know if you have any other questions.
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Best Regards,
Sina Salam