hello am downloading a custom module 
and its gives me error : 
Traceback (most recent call last):
  File "/opt/odoo/odoo-10.0/odoo/http.py", line 641, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/opt/odoo/odoo-10.0/odoo/http.py", line 683, in dispatch
    result = self._call_function(**self.params)
  File "/opt/odoo/odoo-10.0/odoo/http.py", line 333, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/opt/odoo/odoo-10.0/odoo/service/model.py", line 101, in wrapper
    return f(dbname, *args, **kwargs)
  File "/opt/odoo/odoo-10.0/odoo/http.py", line 326, in checked_call
    result = self.endpoint(*a, **kw)
  File "/opt/odoo/odoo-10.0/odoo/http.py", line 941, in __call__
    return self.method(*args, **kw)
  File "/opt/odoo/odoo-10.0/odoo/http.py", line 506, in response_wrap
    response = f(*args, **kw)
  File "/opt/odoo/odoo-10.0/addons/web/controllers/main.py", line 896, in call_button
    action = self._call_kw(model, method, args, {})
  File "/opt/odoo/odoo-10.0/addons/web/controllers/main.py", line 884, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/opt/odoo/odoo-10.0/odoo/api.py", line 689, in call_kw
    return call_kw_multi(method, model, args, kwargs)
  File "/opt/odoo/odoo-10.0/odoo/api.py", line 680, in call_kw_multi
    result = method(recs, *args, **kwargs)
  File "/opt/odoo/odoo-10.0/odoo/addons/base/module/module.py", line 411, in button_immediate_install
    return self._button_immediate_function(type(self).button_install)
  File "/opt/odoo/odoo-10.0/odoo/addons/base/module/module.py", line 485, in _button_immediate_function
    modules.registry.Registry.new(self._cr.dbname, update_module=True)
  File "/opt/odoo/odoo-10.0/odoo/modules/registry.py", line 83, in new
    odoo.modules.load_modules(registry._db, force_demo, status, update_module)
  File "/opt/odoo/odoo-10.0/odoo/modules/loading.py", line 339, in load_modules
    loaded_modules, update_module)
  File "/opt/odoo/odoo-10.0/odoo/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-10.0/odoo/modules/loading.py", line 156, in load_module_graph
    _load_data(cr, module_name, idref, mode, kind='data')
  File "/opt/odoo/odoo-10.0/odoo/modules/loading.py", line 95, in _load_data
    tools.convert_file(cr, module_name, filename, idref, mode, noupdate, kind, report)
  File "/opt/odoo/odoo-10.0/odoo/tools/convert.py", line 845, in convert_file
    convert_xml_import(cr, module, fp, idref, mode, noupdate, report)
  File "/opt/odoo/odoo-10.0/odoo/tools/convert.py", line 915, in convert_xml_import
    obj.parse(doc.getroot(), mode=mode)
  File "/opt/odoo/odoo-10.0/odoo/tools/convert.py", line 796, in parse
    self.parse(rec, mode)
  File "/opt/odoo/odoo-10.0/odoo/tools/convert.py", line 799, in parse
    self._tags[rec.tag](rec, de, mode=mode)
  File "/opt/odoo/odoo-10.0/odoo/tools/convert.py", line 707, in _tag_record
    id = self.env(context=rec_context)['ir.model.data']._update(rec_model, self.module, res, rec_id or False, not self.isnoupdate(data_node), noupdate=self.isnoupdate(data_node), mode=self.mode)
  File "/opt/odoo/odoo-10.0/odoo/addons/base/ir/ir_model.py", line 1236, in _update
    record = record.create(values)
  File "/opt/odoo/odoo-10.0/odoo/addons/base/ir/ir_ui_view.py", line 362, in create
    return super(View, self).create(self._compute_defaults(values))
  File "/opt/odoo/odoo-10.0/odoo/models.py", line 3855, in create
    self._fields[key].determine_inverse(record)
  File "/opt/odoo/odoo-10.0/odoo/fields.py", line 1046, in determine_inverse
    getattr(records, self.inverse)()
  File "/opt/odoo/odoo-10.0/odoo/addons/base/ir/ir_ui_view.py", line 220, in _inverse_arch
    view.write(data)
  File "/opt/odoo/odoo-10.0/odoo/addons/base/ir/ir_ui_view.py", line 380, in write
    return super(View, self).write(self._compute_defaults(vals))
  File "/opt/odoo/odoo-10.0/odoo/models.py", line 3568, in write
    self._write(old_vals)
  File "/opt/odoo/odoo-10.0/odoo/models.py", line 3732, in _write
    self._validate_fields(vals)
  File "/opt/odoo/odoo-10.0/odoo/models.py", line 1081, in _validate_fields
    raise ValidationError("%s\n\n%s" % (_("Error while validating constraint"), tools.ustr(e)))
ParseError: "Error while validating constraint
Field `discount_fixed` does not exist
Error context:
View `pos.order.invoice.extend`
[view_id: 659, xml_id: n/a, model: account.invoice, parent_id: 419]
None" while parsing /opt/odoo/odoo-10.0/addons/fixed_discount_pos/views/account_invoice_view_pos.xml:5, near
<record model="ir.ui.view" id="view_pos_invoice_new_extended">
            <field name="name">pos.order.invoice.extend</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form"/>
            <field name="arch" type="xml">
                <data><xpath expr="//field[@name='discount']" position="after">
                    <field name="discount_fixed" groups="sale.group_discount_per_so_line">discount_fixed</field>
                </xpath>
                <xpath expr="//field[@name='amount_tax']" position="after">
                    <field name="discount_total"/>
                    <field name="discount_percent"/>
                </xpath>
            </data></field>
    </record>and here is the xml:
<?xml version="1.0"?>
<odoo>
<data>
<record model="ir.ui.view" id="view_pos_invoice_new_extended">
<field name="name">pos.order.invoice.extend</field>
<field name="model">account.invoice</field>
<field name="inherit_id" ref="account.invoice_form"/>
<field name="arch" type="xml">
<xpath expr="//field[@name='discount']" position="after">
<field name="discount_fixed" groups="sale.group_discount_per_so_line">discount_fixed</field>
</xpath>
<xpath expr="//field[@name='amount_tax']" position="after">
<field name="discount_total"/>
<field name="discount_percent"/>
</xpath>
</field>
</record>
</data>
</odoo>
am new with this can you please help me to find the error
