No access error during access to Management Console from the Access Portal for an IAM Identity Center's user with AdministratorAccess &/| PowerUserAccess permission sets

The issue you're experiencing suggests that there might be a problem with the permission set assignment or the IAM role creation process in the target AWS account. Here are a few steps you can take to troubleshoot and resolve this issue:

1. Verify the permission set assignment: Make sure that the user or group is correctly assigned to the AWS account and the AdministratorAccess permission set in the IAM Identity Center console. Double-check that the assignment process was completed successfully.

2. Check the IAM role in the target account: The IAM Identity Center should automatically create a corresponding IAM role in the target AWS account when you assign a permission set. Verify that this role exists in the IAM console of the target account and that it has the correct permissions attached.

3. Ensure the IAM Identity Center service-linked role exists: Check that the AWSServiceRoleForSSO service-linked role exists in the target account. This role is necessary for IAM Identity Center to function correctly.

4. Review trust relationship: Examine the trust relationship of the IAM role in the target account to ensure it trusts the IAM Identity Center service.

5. Check for conflicting policies: Look for any Service Control Policies (SCPs) or other organization-level policies that might be restricting access, even for administrators.

6. Verify the identity source: Ensure that the user exists in the identity source connected to IAM Identity Center (e.g., the default directory, Active Directory, or external IdP).

7. Allow time for propagation: Changes in IAM Identity Center can take a few minutes to propagate. Wait for some time and try accessing again.

8. Re-provision the account: If the issue persists, try re-provisioning the account in IAM Identity Center by removing and re-adding the AWS account.

If none of these steps resolve the issue, you may need to contact AWS Support for further assistance, as there might be an underlying problem with the IAM Identity Center configuration or account setup.

Remember that while CloudTrail might not show this specific error, it should still log successful and failed sign-in attempts, which could provide additional context for troubleshooting.

Sources
Sign in to the AWS access portal with your IAM Identity Center credentials - AWS IAM Identity Center
Assign AWS account access for an IAM Identity Center user - AWS IAM Identity Center
AWS IAM Identity Center Features – Amazon Web Services (AWS)

Thanks to the support team, they spotted the real problem in my case. The problem was with an one letter length username. I created another user with two letters as a username and everything is working now. Quite a rare case, I admit. Some validation or better logging on the AWS services side would help spotting the potential problem faster. Let's hope someone can recognize their problem here also.