s3cmd sync necessary user policy, this policy should be attached to user in IAM console

gistfile1.json

{
  "Statement": [
    {
      "Action": [
        "s3:ListAllMyBuckets"
      ],
      "Effect": "Allow",
      "Resource": "arn:aws:s3:::*"
    },
    {
      "Action": [ 
          "s3:ListBucket", 
          "s3:PutObject",
          "s3:PutObjectAcl"
      ],
      "Effect": "Allow",
      "Resource": [
          "arn:aws:s3:::bucketname",
          "arn:aws:s3:::bucketname/*"
      ]
    }
  ]
}