No keys were availble to verify the JWT. TinyMCE RTC

This is the code that I used to produce JWT token for tinymce real time collaboration.

app.post('/jwt', function (req, res) {
  // NOTE: Before you proceed with the TOKEN, verify your users' session or access.
  const payload = {
    sub: '1', // Unique user ID string
    exp: Math.floor(Date.now() / 1000) + (60 * 10) // 10 minutes expiration
  };

  try {
    const token = jwt.sign(payload, privateKey, { algorithm: 'RS256'});
    res.set('content-type', 'application/json');
    res.status(200);
    res.send(JSON.stringify({
      token: token
    }));
  } catch (e) {
    res.status(500);
    res.send(e.message);
  }
});

And code that I used for application is:

const express = require('express');
const jwt = require('jsonwebtoken');
const cors = require('cors');

const app = express();
app.use(cors());
var http = require('http').Server(app);


const privateKey = `
-----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----
`

app.get('/app', function(req, res){
    res.sendFile('D:/Extensions/DV-EDITOR_TESTS' + '/tinymce-rte.html');
});

app.post('/getUserDetails', function(req, res){
  try {    
    res.set('content-type', 'application/json');
    res.status(200);
    res.send(JSON.stringify({
      'username': "TestUser1"
    }));
  } catch (e) {
    res.status(500);
    res.send(e.message);
  }
});

app.post('/getEncryptionKey', function(req, res){
  try {    
    res.set('content-type', 'application/json');
    res.status(200);
    res.send(JSON.stringify({
      'key': key
    }));
  } catch (e) {
    res.status(500);
    res.send(e.message);
  }
});

app.post('/jwt', function (req, res) {
  // NOTE: Before you proceed with the TOKEN, verify your users' session or access.
  const payload = {
    sub: 'userid1', // Unique user ID string
    exp: Math.floor(Date.now() / 1000) + (60 * 10) // 10 minutes expiration
  };

  try {
    const token = jwt.sign(payload, privateKey, { algorithm: 'RS256'});
    res.set('content-type', 'application/json');
    res.status(200);
    res.send(JSON.stringify({
      token: token
    }));
  } catch (e) {
    res.status(500);
    res.send(e.message);
  }
});

http.listen(3000, function(){
    console.log('listening on *:3000');
});

This is a sample application that I am tring to implement before the integration to actual one.

I am new to JWT. So, please help and provide a breif explanation for occurence of this error and its resolution.

But, it is giving this error.