I figured out the answer myself.
In Azure Postgres , we cannot CREATE SUPERUSERS . Since this is a Managed Service , Only Microsoft is part of SUPERUSERS Group
Details in link here
The Azure Database for PostgreSQL server is created with the 3 default roles defined. You can see these roles by running the command: SELECT rolname FROM pg_roles;
your server admin user
Your server admin user is a member of the azure_pg_admin role. However, the server admin account is not part of the azure_superuser role. Since this service is a managed PaaS service, only Microsoft is part of the super user role.