As @C3roe mentioned, the error you're getting is apparently because you're using <script type="module" ... for your script.js. You were doing this because on script.js, you imported addtoCart function from another script (cart.js).

script.js

import { addtocart } from "./cart";            // <-- THIS PART
addtocart(quantity, name, price, imgurl);

//for fun
var fun =0
//loading the html file first cus to get empty spans
document.addEventListener('DOMContentLoaded',()=>{

// ...

One quick fix that can be done is to just redeclare that addtoCart function inside your script.js so you don't have to import it.

script.js

// import { addtocart } from "./cart";             // <-- NO LONGER NEEDED
function addtocart(quantity, name, price, imgurl) {
  const li = document.createElement('li');
  li.textContent = `${name} - ${quantity} x ₹${price} - ${imgurl}`;
  cart.appendChild(li);
}

addtocart(quantity, name, price, imgurl);

//for fun
var fun = 0
//loading the html file first cus to get empty spans
document.addEventListener('DOMContentLoaded', () => {
  //getting all span files as ids
  // ....

Now on your index.html, you don't need to specify type='module' for your script.js.

index.html

<script type="module" src="script.js"></script>  <!-- CHANGE THIS PART -->
<script src="script.js"></script>                <!-- TO BE LIKE THIS -->

I've tested this on my local machine and it doesn't throw the error you specified but instead pops up an alert: