S3 Bucket Policy Issue[API: s3:PutBucketPolicy Access Denied]

The error message you've provided suggests that the IAM entity (user/role) attempting to set the bucket policy does not have the necessary permissions. Specifically, the entity does not have the s3:PutBucketPolicy permission for the given S3 bucket.

Here's how you can resolve this:

• Check the IAM Policy attached to the entity: If it's an IAM User, Role or Group, go to the AWS IAM console and review the permissions attached. Ensure that the entity has the required permission for the S3 bucket in question.

• Example policy to allow s3:PutBucketPolicy on a specific bucket:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "s3:PutBucketPolicy",
            "Resource": "arn:aws:s3:::YOUR_BUCKET_NAME"
        }
    ]
}

If you need to allow the entity to put a bucket policy on all buckets, you can replace the Resource value with "arn:aws:s3:::*".

• S3 Block Public Access: Ensure that you have not enabled S3 Block Public Access at the account level or for the specific bucket. Sometimes, S3 Block Public Access settings can prevent the application of bucket policies that grant public access.

• Assuming Roles: If you're assuming a role (using STS) to execute these actions, make sure that the assumed role has the necessary permissions.

• Service Control Policies (for AWS Organizations): If you're using AWS Organizations and Service Control Policies (SCPs), ensure that the SCPs aren't denying the s3:PutBucketPolicy action.

• Review Bucket Ownership: Make sure that the bucket is owned by your AWS account. If the bucket was transferred to you or created in another account, there might be cross-account permission issues.

• Audit with AWS CloudTrail: If you have CloudTrail set up, you can look into the event logs to get more detailed information about which entity tried accessing and what was the exact reason for denial.

• Explicit Denies: Ensure that there are no explicit Deny statements in any IAM policies attached to the entity that's causing the conflict with the permission.

Once you've determined the cause and adjusted the necessary permissions, try to apply the bucket policy again. If you're still facing issues, consider creating a minimal policy to test the permissions, and then build it up incrementally to achieve your desired configuration.