Datazone Portal unable to sign in using IAM

If you login using the IAM credentials of the principal that you used to create the domain, then you click on the domain URL, you should be auto logged in.

Apart from the previous comment for using IAM credentials. For the preview access of DataZone, for access via portal, you can add the AWS Managed permission policy to the IAM principal to enable portal access for full access control

https://docs.aws.amazon.com/datazone/latest/userguide/security-iam-awsmanpol-AmazonDataZonePortalFullAccessPolicy.html

{ "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": "datazonecontrol:", "Resource": "" } ] }