AWS Transfer for SFTP
User Guide

Amazon S3 Requirements

AWS Transfer for SFTP accesses your Amazon S3 bucket to service your users’ transfer requests, so you will need to provide a bucket when setting up your SFTP server. You can use an existing bucket, or you can create a new one. When you set up your users, you will assign them an IAM role that will determine the level of access you want to provide to your S3 bucket.

See "How Do I Create an S3 Bucket?" to create a new bucket.