I have an IAM Console User with the AdministratorAccess policy attached. IAM Identity Center has been enabled by the root user in the account and have access to IAM Identity Center, but the IAM Console User is unable to access the IAM Identity Center page. When attempting to access the page, the following message is displayed:
IAM Identity Center is already set up in this Region with an organization instance with ARN: arn:aws:sso:::instance/ssoins-{random-numbers}. If you would like to continue with enabling Identity Center in this account, choose Enable.
Why is the IAM Console User with AdministratorAccess unable to see or manage the IAM Identity Center page, while the root user has full access?

Thanks @iBehr. We were so sure we had enabled the IAM Identity Center under the correct account, but after double-checking, you were right. Once we enabled it under the correct account, the IAM Console user was able to manage the IAM Identity Center.