I'm following the official instructions to deploy my strapi starter app to Heroku. The app runs fine locally. The only thing I left out in my deployment instructions were installing the PG node module (it is already installed because my local app uses Postgresql).

Accessing the Heroku logs, I see this:

error: Middleware "strapi::session": App keys are required. 
Please set app.keys in config/server.js (ex: keys: ['myKeyA', 'myKeyB'])

Maybe this is an important detail: I followed this process once, and everything worked. I was able to deploy to Heroku. I tried it again and it didn't work. I was thinking maybe Heroku had a problem with me re-using an app name, but I tried to name the app something different in Heroku and I still had the same error.

Is heroku looking in the wrong place for my server.js file? Should it be looking in my "./config/env/production" folder instead of my "./config" folder?

Per the instructions, here is my ./config/env/production/database.js

const parse = require('pg-connection-string').parse;
const config = parse(process.env.DATABASE_URL);

module.exports = ({ env }) => ({
  connection: {
    client: 'postgres',
    connection: {
      host: config.host,
      port: config.port,
      database: config.database,
      user: config.user,
      password: config.password,
      ssl: {
        rejectUnauthorized: false
      },
    },
    debug: false,
  },
});

Here is my ./config/env/production/server.js

module.exports = ({ env }) => ({
    url: env('MY_HEROKU_URL'),
});

And here is my ./config/server.js

module.exports = ({ env }) => ({
  host: env('HOST', '0.0.0.0'),
  port: env.int('PORT', 1337),
  app: {
    keys: env.array('APP_KEYS'),
  },
});

my package.json for good measure:

{
  "dependencies": {
    "@strapi/plugin-graphql": "^4.0.0",
    "@strapi/plugin-i18n": "4.0.6",
    "@strapi/plugin-users-permissions": "4.0.6",
    "@strapi/strapi": "4.0.6",
    "lodash.set": "^4.3.2",
    "pg": "8.6.0",
    "pg-connection-string": "^2.5.0"
  },
  "name": "backend",
  "private": true,
  "version": "0.1.0",
  "description": "A Strapi application",
  "scripts": {
    "develop": "strapi develop",
    "start": "strapi start",
    "build": "strapi build",
    "strapi": "strapi"
  },
  "devDependencies": {},
  "author": {
    "name": "A Strapi developer"
  },
  "strapi": {
    "uuid": "f64b509e-2d95-4464-8d39-d6f0d1c7a31a",
    "template": "@strapi/template-corporate@^1.0.0",
    "starter": "@strapi/starter-next-corporate"
  },
  "engines": {
    "node": ">=12.x.x <=16.x.x",
    "npm": ">=6.0.0"
  },
  "license": "MIT"
}

I'm running Node v14.18.3 and NPM v6.14.15