IAM role that have permission to unload only one schema in redshift

It is not possible to create an IAM role with permissions specific to a single schema in Amazon Redshift because IAM roles are used to manage permissions for AWS services, and Redshift uses its own internal permission management system for granting access to its objects (databases, schemas, tables, etc.).

To achieve what you're looking for, you should create a new Redshift user with limited permissions and then create a temporary IAM role or credentials that the user can assume to access the necessary S3 bucket for the UNLOAD operation.