Firebase function deploy not working: SyntaxError: Unexpected token '?'

The problem is the same as this question: Firebase Cloud Functions Deploy Error- SyntaxError: Unexpected token '?'

and I've tried all the answers but nothing seems to work.

This is what I get when I try and deploy functions (even the ones that were deploying fine a week ago):

Error: Error occurred while parsing your function triggers.

/Users/islamariful/node_modules/firebase-admin/lib/app/firebase-namespace.js:84
        this.INTERNAL = new FirebaseNamespaceInternals(appStore ?? new lifecycle_1.AppStore());
                                                                 ^

SyntaxError: Unexpected token '?'
    at wrapSafe (internal/modules/cjs/loader.js:1052:16)
    at Module._compile (internal/modules/cjs/loader.js:1100:27)
    at Module._compile (pkg/prelude/bootstrap.js:1394:32)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1156:10)
    at Module.load (internal/modules/cjs/loader.js:984:32)
    at Function.Module._load (internal/modules/cjs/loader.js:877:14)
    at Module.require (internal/modules/cjs/loader.js:1024:19)
    at Module.require (pkg/prelude/bootstrap.js:1338:31)
    at require (internal/modules/cjs/helpers.js:72:18)
    at Object.<anonymous> (/Users/islamariful/node_modules/firebase-admin/lib/default-namespace.js:19:30)

This is my package.json file:

 {
  "name": "functions",
  "description": "Cloud Functions for Firebase",
  "scripts": {
    "lint": "eslint .",
    "serve": "firebase emulators:start --only functions",
    "shell": "firebase functions:shell",
    "start": "npm run shell",
    "deploy": "firebase deploy --only functions",
    "logs": "firebase functions:log"
  },
  "engines": {
    "node": "16"
  },
  "main": "index.js",
  "dependencies": {
    "axios": "^0.21.1",
    "firebase-admin": "^9.8.0",
    "firebase-functions": "^3.14.1",
    "pdfmake": "^0.2.2"
  },
  "devDependencies": {
    "eslint": "^7.6.0",
    "eslint-config-google": "^0.14.0",
    "eslint-plugin-jsdoc": "^36.0.7",
    "firebase-functions-test": "^0.2.0"
  },
  "private": true
}

I've tried multiple things but still can't get this to work. Any ideas on how I can fix this?