AWS:S3 NoSuchKey Error

I am getting HTTP 404 response when trying to serve my files from s3 bucket. Files (objects) exist at that particular directory of my bucket but still I'am getting the HTTP 404 response. When I try to load that link , I get this error Syntax from AWS

This XML file does not appear to have any style information associated with it. The document tree is shown below.
<Error>
<Code>NoSuchKey</Code>
<Message>The specified key does not exist.</Message>
<Key>assets/assets/bootstrap/css/bootstrap.css</Key>
<RequestId>B4B83B5DB304478F</RequestId>
<HostId>
Ydl9LiDDiQqr6BxVhHGHWTkbnJm922zuajgaOFj8UMpM13Qv7KZPbv17jauLj8XdoG/8uKX40HA=
</HostId>
</Error> 

I am using BOTO to handle my static storages. I have checked the ACCESS_KEY and SECRET_KEY, they are correct but still I'm getting 404 response.

How can I setup my S3 bucket so that It functions perfectly?

My CORS configuration is this:

<?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="http://ift.tt/1f8lKAh">
    <CORSRule>
        <AllowedOrigin>*</AllowedOrigin>
        <AllowedMethod>GET</AllowedMethod>
        <AllowedMethod>POST</AllowedMethod>
        <AllowedMethod>PUT</AllowedMethod>
        <MaxAgeSeconds>3000</MaxAgeSeconds>
        <AllowedHeader>Authorization</AllowedHeader>
    </CORSRule>
</CORSConfiguration>