Root user can't create OU

If you're unable to create an Organizational Unit (OU) as the root user, there are a few things you should check:

1. Verify that you are indeed signed in as the root user of the management account. Only the management account has the ability to create OUs in AWS Organizations.

2. Ensure that AWS Organizations is properly set up in your account. If it's not, you'll need to create an organization first before you can create OUs.

3. Check if your account has been verified. New AWS accounts sometimes have limited functionality until they're fully verified, which might include restrictions on creating OUs.

4. If your account is very new (created within the last day), try starting a free tier EC2 instance (Amazon Linux AMI is fine) and wait a few minutes. Sometimes new accounts need a small amount of activity before all features are fully enabled.

5. Look for any error messages when attempting to create the OU. If you see any, they might provide more specific information about the issue.

6. Check your AWS CloudTrail logs for any permission-related errors that might be preventing the OU creation.

If none of these steps resolve the issue, it's recommended to contact AWS Support for further assistance, as they can look into account-specific details that might be causing the problem.

Remember, creating OUs should typically be possible for the root user of the management account, so if you're still unable to do so after these checks, there may be an underlying issue with your account that requires AWS Support intervention.
Sources
Creating an organizational unit (OU) with AWS Organizations - AWS Organizations
Cannot create accounts in my organization | AWS re:Post

In my case I realized you have to click the "Root" folder of the accounts first in the Accounts list rather than ticking an individual account.

Then it is no longer greyed out.