How do I find the reason a domain registration failed

When you encounter the message "We can't finish registering your domain. Contact AWS Support..." in Route 53, unfortunately there isn't a way for you to directly access more detailed error information about why the registration failed. This is a common frustration as the system often provides only generic error messages without specific details.

Since you've already verified that:

• The domains are available
• Your registration information is correct
• You've tried different domain names and TLDs
• The domain names don't contain special characters

And the failure happens quickly before email/phone verification, your best course of action is to contact AWS Support. For domain registration issues, you can do this regardless of your support plan level:

1. Sign in to the AWS Support Center using the same AWS account you're using for domain registration
2. Select "Account and Billing Support" as the category
3. Choose "Domains" as the service
4. Select "Registration Issue" as the category
5. Provide a detailed description of your issue, including all the troubleshooting steps you've already taken
6. Choose "Web" as the contact method

While waiting for AWS Support to respond (which can sometimes take a few business days), there aren't additional self-service troubleshooting options available to determine the specific cause of the registration failure. You'll need to wait for AWS Support to investigate your specific account and registration attempts to identify the underlying issue.
Sources
Unable to register a new domain | AWS re:Post
Contacting AWS Support about domain registration issues - Amazon Route 53
Route 53 - Received message - We can't finish registering your domain | AWS re:Post
Route53: Wasn't able to register domain name | AWS re:Post