When you failed to assign a domain

You will not be allowed assign your domain name to your project if you have following settings on your AWS account, our products or other AWS accounts.

CloudFront uses it as its CNAME.

Solution: Detach it from CNAME.
For more details, please check AWS' official document: Adding and Moving Alternate Domain Names (CNAMEs) .

AWS S3 uses it as a bucket name with Website hosting enabled.

Solution: Disable Webhosting on S3
For more detail, please check AWS' official document: How Do I Configure an S3 Bucket for Static Website Hosting? .