Why won't Storage Studio save my S3 Express One Zone bucket?
Short answer: Uploads to S3 Express One Zone buckets don't work with Storage Studio, and Express keeps only one copy, so it isn't suited to backups. Names ending in --x-s3 are refused. Create a normal S3 bucket instead.
Because uploads to S3 Express One Zone buckets don't work with Storage Studio, and Express isn't suitable for backups anyway — it keeps a single copy in one zone and costs more. Bucket names ending in --x-s3 are Express buckets, so the form refuses to save them.
Create a normal S3 bucket in the same region and use that instead. If an older account already points at Express, its details show an S3 Express One Zone — upload not supported notice; edit the account to use a standard bucket.