Heroku app works locally but not when deployed

Question

I am trying to upload an app to heroku, but I can't seem to figure out why my app will work perfectly locally either via nodemon or heroku local but it will show "internal server error" once deployed to heroku. It seems to be something to do with ejs or maybe file paths?

This is what the logs on heroku show.

2019-12-10T11:01:15.750370+00:00 app[web.1]: Error: /app/views/home.ejs:1
2019-12-10T11:01:15.750381+00:00 app[web.1]:  >> 1| <%- include("partials/header")-%>
2019-12-10T11:01:15.750383+00:00 app[web.1]:     2| 
2019-12-10T11:01:15.750385+00:00 app[web.1]:     3| <div class="container-fluid" id="homePage">
2019-12-10T11:01:15.750386+00:00 app[web.1]:     4| 
2019-12-10T11:01:15.750387+00:00 app[web.1]: 
2019-12-10T11:01:15.750389+00:00 app[web.1]: Could not find the include file "partials/header"
2019-12-10T11:01:15.750390+00:00 app[web.1]:     at getIncludePath (/app/node_modules/ejs/lib/ejs.js:165:13)
2019-12-10T11:01:15.750391+00:00 app[web.1]:     at includeFile (/app/node_modules/ejs/lib/ejs.js:291:19)
2019-12-10T11:01:15.750392+00:00 app[web.1]:     at include (/app/node_modules/ejs/lib/ejs.js:680:16)
2019-12-10T11:01:15.750394+00:00 app[web.1]:     at eval (/app/views/home.ejs:10:17)
2019-12-10T11:01:15.750395+00:00 app[web.1]:     at home (/app/node_modules/ejs/lib/ejs.js:682:17)
2019-12-10T11:01:15.750396+00:00 app[web.1]:     at tryHandleCache (/app/node_modules/ejs/lib/ejs.js:254:36)
2019-12-10T11:01:15.750398+00:00 app[web.1]:     at View.exports.renderFile [as engine] (/app/node_modules/ejs/lib/ejs.js:485:10)
2019-12-10T11:01:15.750400+00:00 app[web.1]:     at View.render (/app/node_modules/express/lib/view.js:135:8)
2019-12-10T11:01:15.750401+00:00 app[web.1]:     at tryRender (/app/node_modules/express/lib/application.js:640:10)
2019-12-10T11:01:15.750402+00:00 app[web.1]:     at Function.render (/app/node_modules/express/lib/application.js:592:3)
2019-12-10T11:01:15.745815+00:00 heroku[router]: at=info method=GET path="/" host=voila-va.herokuapp.com request_id=4fca84a8-f079-4b85-8eae-860887f3de01 fwd="212.139.199.181" dyno=web.1 connect=1ms service=3ms status=500 bytes=404 protocol=https

This is my app.js

//jshint esversion:6

const express = require("express");
const bodyParser = require("body-parser");
const ejs = require("ejs");
const nodemailer = require('nodemailer');
// const exphbs = require("express-handlebars");#
const csp = require('express-csp-header');
const app = express();


app.set('view engine', 'ejs');
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({extended: true}));
app.use(express.static("public"));
// app.use(csp({
//     policies: {
//         'default-src': [csp.SELF],
//         'script-src': [csp.SELF, csp.INLINE, 'somehost.com'],
//         'style-src': [csp.SELF, 'mystyles.net'],
//         'img-src': ['data:', 'images.com'],
//         'worker-src': [csp.NONE],
//         'block-all-mixed-content': true
//     }
// }));

app.get("/",function(req,res){
  res.render("home");
});

app.get("/services",function(req,res){
  res.render("services");
});

app.get("/pricing",function(req,res){
  res.render("pricing");
});

app.get("/about",function(req,res){
  res.render("about");
});
app.get("/contact",function(req,res){
  res.render("contact");
});

app.get("/ec",function(req,res){
  res.render("enquirycomplete");
});

app.get("/ef",function(req,res){
  res.render("enquiryfailed");
});

app.get("/privacypolicy",function(req,res){
  res.render("privacypolicy");
});


app.post("/send",function(req,res){


  const transporter = nodemailer.createTransport({
      host: 'smtp.ethereal.email',
      port: 587,
      secure: false, // true for 465, false for other ports
      auth: {
          user: 'margie.bartell@ethereal.email', // generated ethereal user
          pass: 'bSEaH13htNzHjkPZEn' // generated ethereal password
      }

  });
  const output = {
    h2:"<h2>You've got a new enquiry!</h2>",
    firstName: req.body.first_name,
    lastName: req.body.last_name,
    message: req.body.comments,
    telephone: req.body.telephone
  };
  const htmlForm =
  `<h2>You've got a new enquiry!</h2>
  <p><%=output.firstName%></p>
  <p><%=output.lastName%></p>
  <p><%=output.message%></p>
  <p><%=output.telephone</p>`;

    const mailOptions = {
        from: req.body.email, // sender address
        to: 'danieltaylor236@gmail.com', // list of receivers
        subject: 'Voila Enquiry',
        text: req.body.comments, // Subject line
        // html: output, // plain text body
        replyTo: req.body.email
    };
    transporter.sendMail(mailOptions,function(error,info){
      if (error){
         console.log("this is not working" + error);
      }else{
        console.log("EmailForm sent!" + info.response);
      }
  });
  transporter.verify(function(error, success) {
  if (error) {
    console.log(error);
    res.render("enquiryfailed");
  } else {
    console.log("Server is ready to take our messages");
    res.render("enquirycomplete");
    console.log(mailOptions);
  }
});

});



app.listen(process.env.PORT || 5000,function(){
  console.log("server started on port 3000");
});

This is my package.json

{
  "name": "voila-website",
  "version": "1.0.0",
  "description": "Voila Website",
  "main": "app.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [
    "heroku"
  ],
  "repository": {
    "type": "git",
    "url": "git+https://github.com/Tayho25/Voila-website.git"
  },
  "author": "netdoodle",
  "license": "ISC",
  "bugs": {
    "url": "https://github.com/Tayho25/Voila-website/issues"
  },
  "homepage": "https://github.com/Tayho25/Voila-website#readme",
  "dependencies": {
    "body-parser": "^1.19.0",
    "ejs": "^2.7.1",
    "express": "^4.17.1",
    "express-csp-header": "^2.3.2",
    "nodemailer": "^6.3.1"
  },
  "engines": {
    "node": "v12.13.1"
  }
}