Error While Creating Redshift Cluster with IAM User

Hello.

What specific settings are you trying to create a Redshift cluster with?
Judging from the error message, it seems that the IAM role that is set when creating the RedShift cluster, not the IAM user, does not have sufficient permissions to access SecretsManager.
Please try creating an IAM role for Redshift using the steps in the document below and attaching it when creating a cluster.
https://docs.aws.amazon.com/redshift/latest/mgmt/default-iam-role.html

Specifically, you can specify IAM roles in the following items when creating a cluster.