'qty_available' is a computed field (in real time). Its calculations requires a user to be at least an employee. This is why public / portal users can't observe such a page.

You may show quantity under sudo():

<span t-field="product.sudo().qty_available"/>

In such a way the figure would be calculated as you are under Odoo SuperUser. Take into account that this approach might have drawbacks:

1. It is not totally safe, altough there is no much space to retrieve extra information here

2. In case you have some restrictions by locations, they would not be applied: a user would see ALL stocks.

As alternative you can develop an own method to calculate available quantities. In that method you can also apply sudo(), but it is possible to do that more carefully in the method.