HTML button not clickable despite adding onclick listener

I’ve created a “+” button to send text from an input field to my Firebase database. However, the result is not showing up in that database, and the “+” button is not clickable 🙁 Please take a look at my code below.

index.html:

<!doctype html>
<html>
    <head>
        <title>Reel Time</title>
        <link rel="preconnect" href="https://fonts.googleapis.com">
        <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
        <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300&family=Ultra&display=swap" rel="stylesheet">
        <link rel="stylesheet" href="index.css">
    </head>
    <body>
        <div class="container">
            <h1>Reel Time</h1>
            <div class="field">
                <input type="text" id="input-field" placeholder="Pulp Fiction">
                <button id="add-button">+</button>
            </div>
        </div>
        <script src="functions.js" type="module"></script>
        <script src="index.js" type="module"></script>
    </body>
</html>

index.js:

import { initializeApp } from "https://www.gstatic.com/firebasejs/9.15.0/firebase-app.js"
import { getDatabase, ref, push } from "https://www.gstatic.com/firebasejs/9.15.0/firebase-database.js"

const appSettings = {
    databaseURL: "https://playground-91206-default-rtdb.asia-southeast1.firebasedatabase.app/"
}

const app = initializeApp(appSettings)
const database = getDatabase(app)
const moviesInDB = ref(database, "movies")

const inputFieldEl = document.getElementById("input-field")
const addButtonEl = document.getElementById("add-button")

addButtonEl.addEventListener("click", function() {
    let inputValue = inputFieldEl.value
    
    push(moviesInDB, inputValue)
    
    console.log(`${inputValue} added to database`)
})

functions.js:

export function add(a,b) {
   return a + b
}

Please advise 🙂