In openerp you make a request to read the assets while the transaction creating is not done and As the assets are generated during the same transaction and the rendering of the templates will call them. There is a scenario where the assets are unreachable: when you make a request to read the assets while the transaction creating them is not done. Indeed, when you make an asset request, the controller has to read the 'ir.attachment' table.
For this use below code in .py file,

def get_pdf(self, cr, uid, ids, report_name, html=None, data=None, context=None):

    if not config['test_enable']:
        context = dict(context, commit_assetsbundle=True)

    if html is None:
        html = self.get_html(cr, uid, ids, report_name, data=data, context=context)

    # The test cursor prevents the use of another environnment while the current
    # transaction is not finished, leading to a deadlock when the report requests
    # an asset bundle during the execution of test scenarios. In this case, return
    # the html version.
    if isinstance(cr, TestCursor):
        return html

    html = html.decode('utf-8')  # Ensure the current document is utf-8 encoded.

    # Get the ir.actions.report.xml record we are working on.
    report = self._get_report_from_name(cr, uid, report_name)
    # Check if we have to save the report or if we have to get one from the db.
    save_in_attachment = self._check_attachment_use(cr, uid, ids, report)
    # Get the paperformat associated to the report, otherwise fallback on the company one.
    if not report.paperformat_id:
        user = self.pool['res.users'].browse(cr, uid, uid)
        paperformat = user.company_id.paperformat_id
    else:
        paperformat = report.paperformat_id

    # Preparing the minimal html pages
    headerhtml = []
    contenthtml = []
    footerhtml = []
    irconfig_obj = self.pool['ir.config_parameter']
    base_url = irconfig_obj.get_param(cr, SUPERUSER_ID, 'report.url') or irconfig_obj.get_param(cr, SUPERUSER_ID, 'web.base.url')

    # Minimal page renderer
    view_obj = self.pool['ir.ui.view']
    render_minimal = partial(view_obj.render, cr, uid, 'report.minimal_layout', context=context)

    # The received html report must be simplified. We convert it in a xml tree
    # in order to extract headers, bodies and footers.
    try:
        root = lxml.html.fromstring(html)
        match_klass = "//div[contains(concat(' ', normalize-space(@class), ' '), ' {} ')]"

        for node in root.xpath(match_klass.format('header')):
            body = lxml.html.tostring(node)
            header = render_minimal(dict(subst=True, body=body, base_url=base_url))
            headerhtml.append(header)

        for node in root.xpath(match_klass.format('footer')):
            body = lxml.html.tostring(node)
            footer = render_minimal(dict(subst=True, body=body, base_url=base_url))
            footerhtml.append(footer)

        for node in root.xpath(match_klass.format('page')):
            # Previously, we marked some reports to be saved in attachment via their ids, so we
            # must set a relation between report ids and report's content. We use the QWeb
            # branding in order to do so: searching after a node having a data-oe-model
            # attribute with the value of the current report model and read its oe-id attribute
            if ids and len(ids) == 1:
                reportid = ids[0]
            else:
                oemodelnode = node.find(".//*[@data-oe-model='%s']" % report.model)
                if oemodelnode is not None:
                    reportid = oemodelnode.get('data-oe-id')
                    if reportid:
                        reportid = int(reportid)
                else:
                    reportid = False

            # Extract the body
            body = lxml.html.tostring(node)
            reportcontent = render_minimal(dict(subst=False, body=body, base_url=base_url))

            contenthtml.append(tuple([reportid, reportcontent]))

    except lxml.etree.XMLSyntaxError:
        contenthtml = []
        contenthtml.append(html)
        save__attachment = {}  # Don't save this potentially malformed document

    # Get paperformat arguments set in the root html tag. They are prioritized over
    # paperformat-record arguments.
    specific_paperformat_args = {}
    for attribute in root.items():
        if attribute[0].startswith('data-report-'):
            specific_paperformat_args[attribute[0]] = attribute[1]

    # Run wkhtmltopdf process
    return self._run_wkhtmltopdf(
        cr, uid, headerhtml, footerhtml, contenthtml, context.get('landscape'),
        paperformat, specific_paperformat_args, save_in_attachment,
        context.get('set_viewport_size')
    )