Hi,

1- The value of your href must be a link to a controller like the below code:

<a role="button" class="btn btn-primary d-none d-md-inline-block" href="/shop/print" target="_blank" aria-label="Print" title="Print"><i class="fa fa-print"></i> Print</a>

2- After, create a controller like the following code :

@http.route(['/shop/print'], type='http', auth="public", website=True)
def print_saleorder(self, **kwargs):
    sale_order_id = request.session.get('sale_last_order_id')
    if sale_order_id:
        pdf, _ = request.env.ref('sale.action_report_saleorder').sudo().render_qweb_pdf([sale_order_id])
        pdfhttpheaders = [('Content-Type', 'application/pdf'), ('Content-Length', u'%s' % len(pdf))]
        return request.make_response(pdf, headers=pdfhttpheaders)
    else:
        return request.redirect('/shop')

Best regards!