I got this message while trying to install Expense Tracker. Can anyone advise on the reason for this error? Thanks.

Odoo Server Error

Traceback (most recent call last):
  File "/opt/odoo/odoo-server/openerp/http.py", line 646, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/opt/odoo/odoo-server/openerp/http.py", line 683, in dispatch
    result = self._call_function(**self.params)
  File "/opt/odoo/odoo-server/openerp/http.py", line 319, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/service/model.py", line 118, in wrapper
    return f(dbname, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/http.py", line 312, in checked_call
    result = self.endpoint(*a, **kw)
  File "/opt/odoo/odoo-server/openerp/http.py", line 962, in __call__
    return self.method(*args, **kw)
  File "/opt/odoo/odoo-server/openerp/http.py", line 512, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 901, in call_button
    action = self._call_kw(model, method, args, {})
  File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 889, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/api.py", line 250, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/addons/base/module/module.py", line 459, in button_immediate_install
    return self._button_immediate_function(cr, uid, ids, self.button_install, context=context)
  File "/opt/odoo/odoo-server/openerp/api.py", line 250, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/addons/base/module/module.py", line 533, in _button_immediate_function
    registry = openerp.modules.registry.RegistryManager.new(cr.dbname, update_module=True)
  File "/opt/odoo/odoo-server/openerp/modules/registry.py", line 386, in new
    openerp.modules.load_modules(registry._db, force_demo, status, update_module)
  File "/opt/odoo/odoo-server/openerp/modules/loading.py", line 338, in load_modules
    loaded_modules, update_module)
  File "/opt/odoo/odoo-server/openerp/modules/loading.py", line 237, in load_marked_modules
    loaded, processed = load_module_graph(cr, graph, progressdict, report=report, skip_modules=loaded_modules, perform_checks=perform_checks)
  File "/opt/odoo/odoo-server/openerp/modules/loading.py", line 156, in load_module_graph
    _load_data(cr, module_name, idref, mode, kind='data')
  File "/opt/odoo/odoo-server/openerp/modules/loading.py", line 98, in _load_data
    tools.convert_file(cr, module_name, filename, idref, mode, noupdate, kind, report)
  File "/opt/odoo/odoo-server/openerp/tools/convert.py", line 851, in convert_file
    convert_xml_import(cr, module, fp, idref, mode, noupdate, report)
  File "/opt/odoo/odoo-server/openerp/tools/convert.py", line 938, in convert_xml_import
    obj.parse(doc.getroot(), mode=mode)
  File "/opt/odoo/odoo-server/openerp/tools/convert.py", line 801, in parse
    self.parse(rec, mode)
  File "/opt/odoo/odoo-server/openerp/tools/convert.py", line 804, in parse
    self._tags[rec.tag](self.cr, rec, de, mode=mode)
  File "/opt/odoo/odoo-server/openerp/tools/convert.py", line 503, in _tag_menuitem
    menu_parent_id = self.id_get(cr, rec.get('parent',''))
  File "/opt/odoo/odoo-server/openerp/tools/convert.py", line 782, in id_get
    res = self.model_id_get(cr, id_str, raise_if_not_found)
  File "/opt/odoo/odoo-server/openerp/tools/convert.py", line 793, in model_id_get
    raise_if_not_found=raise_if_not_found)
  File "/opt/odoo/odoo-server/openerp/api.py", line 250, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/addons/base/ir/ir_model.py", line 983, in xmlid_to_res_model_res_id
    return self.xmlid_lookup(cr, uid, xmlid)[1:3]
  File "/opt/odoo/odoo-server/openerp/api.py", line 250, in wrapper
    return old_api(self, *args, **kwargs)
  File "<string>", line 2, in xmlid_lookup
  File "/opt/odoo/odoo-server/openerp/tools/cache.py", line 85, in lookup
    value = d[key] = self.method(*args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/addons/base/ir/ir_model.py", line 973, in xmlid_lookup
    raise ValueError('External ID not found in the system: %s' % (xmlid))
ParseError: "External ID not found in the system: base.menu_ir_cron" while parsing /opt/odoo/custom/addons/crontab_config/crontab_config_view.xml:94, near
<menuitem action="action_crontab_config" id="menu_crontab_config" parent="base.menu_ir_cron" sequence="11"/>