question

First of all, your bot access token has the right to read posts and attachments under a team/chat/conversation the bot is a member of.

In your case, I don't see the code nor the environment so it's hard for me to tell what could be wrong. This is how I could successfully download a binary file attachment in my bot using Node JS.

const url = require('url')
try{
  var attachmentUri = "https://dl.mvp.ringcentral.com/file/NNN"
  var accessToken = "Valid-AccessToken"
  download(attachmentUri, accessToken, (err, content) => {
     console.log("Done")
  })
}catch(e){
  console.log(e)
}
var https = require('https');
const download = function(attachmentUri, token, cb) {
  var file = fs.createWriteStream("testing.png");
  var u = url.parse(attachmentUri);
  var options = {
    host: u.host,
    path: u.pathname,
    method: "GET",
    headers: {
        Authorization: `Bearer ${token}`
      }
    }

  const req = https.request(options, res => {
    var chunks = [];
    res.setEncoding('binary');
    res.on('data', (chunk) => {
      chunks += chunk;
    }).on('end', () => {
      file.write(chunks, 'binary');
      file.on('finish', () => {
        console.log('File Saved !');
    });
    res.pipe(file);
    })
  })
  req.on('error', error => {
    console.error(error)
  })
  req.end()
}

If you believe that your code is correct and you still receive the error, I recommend to submit a dev support ticket so you can provide more details for the investigation.