From amazon documentation:
https://docs.aws.amazon.com/AmazonS3/latest/dev/BucketRestrictions.html

Rules for Bucket Naming

A bucket name must be unique across all existing bucket names in Amazon S3. We recommend that all bucket names comply with DNS naming conventions. These conventions are enforced in all Regions except for the US East (N. Virginia) Region.

If you use the AWS Management Console, bucket names must be DNS-compliant in all Regions.

Important

On March 1, 2018, we are updating our naming conventions for S3 buckets in the US East (N. Virginia) Region to match the naming conventions we use in all other worldwide AWS Regions. After this date, Amazon S3 will no longer support creating bucket names that contain uppercase letters or underscores. This change ensures that each bucket can be addressed using virtual host style addressing, such as https://myawsbucket.s3.amazonaws.com. We highly recommend that you review your existing bucket-creation processes to ensure your adherence to our DNS-compliant naming conventions.