How do I resolve the ERROR: S3 error: 404 (NoSuchKey) when using s3cmd?

I tested this out by on one of my existing buckets as well as a new bucket, with the same json file (though I formatted mine as a txt file.)

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Deny",
    "Principal": "*",
    "Action": "s3:*",
    "Resource": "arn:aws:s3:::bucket-policy-example/*",
    "Condition": {
      "NotIpAddress": {
        "aws:SourceIp": "172.104.2.4"
      }
    }
  }]
}

When I ran:

s3cmd setpolicy bucket-policy-test.txt s3://mybucket

In both cases I did not run into the error, and the bucket policy was applied. For example:

s3cmd setpolicy bucket-policy-test.txt s3://mybucket
~ % s3cmd info s3://mybucket
s3://mybucket/ (bucket):
   Location:  default
   Payer:     BucketOwner
   Expiration Rule: none
   Policy:    b'{\n  "Version": "2012-10-17",\n  "Statement": [{\n    "Effect": "Deny",\n    "Principal": "*",\n    "Action": "s3:*",\n    "Resource": "arn:aws:s3:::bucket-policy-example/*",\n    "Condition": {\n      "NotIpAddress": {\n        "aws:SourceIp": "172.104.2.4"\n      }\n    }\n  }]\n}\n'

The first thing to check might be whether your json is valid using a json validator.

Aside from that, in doing some research on this, the most helpful resource I came across was this:

https://aws.amazon.com/premiumsupport/knowledge-center/404-error-nosuchkey-s3/

Based on this, it could have something to do with the syntax of the command, the file path or special characters used.