Hi,

You can override the function of 'Confirm' button in sale order and create invoice from it.
See the below code.

class SaleOrder(models.Model):
    _inherit = 'sale.order'

    def action_confirm(self):
        res = super(SaleOrder, self).action_confirm()
        invoice_lines = []
        for line in self.order_line:
            vals = {
                'name': line.name,
                'price_unit': line.price_unit,
                'quantity': line.product_uom_qty,
                'product_id': line.product_id.id,
                'product_uom_id': line.product_uom.id,
                'tax_ids': [(6, 0, line.tax_id.ids)],
                'sale_line_ids': [(6, 0, [line.id])],
            }
            invoice_lines.append((0, 0, vals))
        self.env['account.move'].create({
            'ref': self.client_order_ref,
            'move_type': 'out_invoice',
            'invoice_origin': self.name,
            'invoice_user_id': self.user_id.id,
            'partner_id': self.partner_invoice_id.id,
            'currency_id': self.pricelist_id.currency_id.id,
            'invoice_line_ids': invoice_lines
        })
        return res

Regards