Hi,

You can create popup using below code.

<odoo>

    <template name="" id="q_return_button_id"

              inherit_id="website_sale.payment">

        <xpath expr="//t[@t-call='payment.checkout']" position="after">

            <div id="upi_payment">

                <button name="get_qr_code" id="upi_payment"

                        class="btn btn-primary" custom="payment"

                        data-bs-toggle="modal"

                        data-bs-target="#exampleModalCenter">Payment

                </button>

           </div>

<!-- Button trigger modal -->

<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#exampleModalCenter">

  Launch demo modal

</button>

<!-- Modal -->

<div class="modal fade" id="exampleModalCenter" tabindex="-1" role="dialog" aria-labelledby="exampleModalCenterTitle" aria-hidden="true">

  <div class="modal-dialog modal-dialog-centered" role="document">

    <div class="modal-content">

      <div class="modal-header">

        <h5 class="modal-title" id="exampleModalLongTitle">Modal title</h5>

        <button type="button" class="close" data-dismiss="modal" aria-label="Close">

          <span aria-hidden="true">&times;</span>

        </button>

      </div>

      <div class="modal-body">

        ...

      </div>

      <div class="modal-footer">

        <button type="button" class="btn btn-secondary" data-dismiss="modal">Close</button>

        <button type="button" class="btn btn-primary">Save changes</button>

      </div>

    </div>

  </div>

</div>

        </xpath>

    </template>

</odoo>

if you are using odoo V16,then you need to change "data-toggle" to "data-bs-toggle"

Regards