Pull up a web browser and go to http://example-bucket.s3.amazonaws.com, substituting your desired bucket name for example-bucket. If you get any response other than an XML error that says NoSuchBucket, then the bucket name is already taken.
The bucket namespace is global (not per-account), so if someone else already has a bucket by that name, you won't be able to create it.
The workaround is to create a bucket with a different name, configure it the way you want for the redirect, make a note of the bucket's web site hosting endpoint, create a CloudFront distribution, configure the origin domain name as the web site hosting endpoint for the new bucket, configure the CloudFront Alternate Domain Name as your original name (the hostname you want to be able to redirect), and then point the DNS to the assigned CloudFront endpoint. This configuration allows you to have a bucket whose name does not need to match the CNAME record, because CloudFront handles the remapping automatically. (These steps probably sound complicated, but once you've done it, it all makes sense).
An advantage of this solution is that you can also associate a certificate from Amazon Certificate Manager so that https access to the hostname you want to redirect works, as well as http. (When you use S3 directly, the redirection feature only supports http).
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