Why do I get "Access Denied" when listing S3 buckets?
Short answer: With a least-privilege user that's expected: it can't list every bucket in your AWS account, only use its own. Enter that bucket's exact name. If reads or writes inside it are denied too, fix the user's policy in AWS.
That's expected with a least-privilege user. A user limited to one bucket can't list every bucket in your AWS account — that's the safe setup. Work with your specific bucket, which it can read and write.
Make sure the Bucket field holds that bucket's exact name. If you also get Access Denied when reading or writing inside the bucket, the user's permissions don't include that bucket — check the policy in the AWS console.