I was trying to append a custom field called pos_brand from the product form (in product.template) onto the POS orderline. Using this code

<?xml version="1.0" encoding="UTF-8"?>
<templates id="point_of_sale.template" xml:space="preserve">
    <t t-extend="Orderline">
        <t t-jquery=".info-list" t-operation="append">
            <t t-if="line.get_product().pos_brand!=='False'">
                <li class="info orderline-brand">
                    <i class='fa fa-tag' role="img" aria-label="Brand" title="Brand"/><t t-esc="line.get_product().pos_brand" />
                </li>
            </t>
        </t>
    </t>
</templates>

And this is the error that followed by.

Error:
Odoo Server Error

Traceback (most recent call last):
File "/home/Robert/Desktop/odoo-12.0/odoo/http.py", line 656, in _handle_exception
return super(JsonRequest, self)._handle_exception(exception)
File "/home/Robert/Desktop/odoo-12.0/odoo/http.py", line 314, in _handle_exception
raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
File "/home/Robert/Desktop/odoo-12.0/odoo/tools/pycompat.py", line 87, in reraise
raise value
File "/home/Robert/Desktop/odoo-12.0/odoo/http.py", line 698, in dispatch
result = self._call_function(**self.params)
File "/home/Robert/Desktop/odoo-12.0/odoo/http.py", line 346, in _call_function
return checked_call(self.db, *args, **kwargs)
File "/home/Robert/Desktop/odoo-12.0/odoo/service/model.py", line 97, in wrapper
return f(dbname, *args, **kwargs)
File "/home/Robert/Desktop/odoo-12.0/odoo/http.py", line 339, in checked_call
result = self.endpoint(*a, **kw)
File "/home/Robert/Desktop/odoo-12.0/odoo/http.py", line 941, in __call__
return self.method(*args, **kw)
File "/home/Robert/Desktop/odoo-12.0/odoo/http.py", line 519, in response_wrap
response = f(*args, **kw)
File "/home/Robert/Desktop/odoo-12.0/addons/web/controllers/main.py", line 966, in call_button
action = self._call_kw(model, method, args, {})
File "/home/Robert/Desktop/odoo-12.0/addons/web/controllers/main.py", line 954, in _call_kw
return call_kw(request.env[model], method, args, kwargs)
File "/home/Robert/Desktop/odoo-12.0/odoo/api.py", line 759, in call_kw
return _call_kw_multi(method, model, args, kwargs)
File "/home/Robert/Desktop/odoo-12.0/odoo/api.py", line 746, in _call_kw_multi
result = method(recs, *args, **kwargs)
File "<decorator-gen-67>", line 2, in button_immediate_upgrade
File "/home/Robert/Desktop/odoo-12.0/odoo/addons/base/models/ir_module.py", line 74, in check_and_log
return method(self, *args, **kwargs)
File "/home/Robert/Desktop/odoo-12.0/odoo/addons/base/models/ir_module.py", line 622, in button_immediate_upgrade
return self._button_immediate_function(type(self).button_upgrade)
File "/home/Robert/Desktop/odoo-12.0/odoo/addons/base/models/ir_module.py", line 561, in _button_immediate_function
modules.registry.Registry.new(self._cr.dbname, update_module=True)
File "/home/Robert/Desktop/odoo-12.0/odoo/modules/registry.py", line 86, in new
odoo.modules.load_modules(registry._db, force_demo, status, update_module)
File "/home/Robert/Desktop/odoo-12.0/odoo/modules/loading.py", line 417, in load_modules
force, status, report, loaded_modules, update_module, models_to_check)
File "/home/Robert/Desktop/odoo-12.0/odoo/modules/loading.py", line 313, in load_marked_modules
perform_checks=perform_checks, models_to_check=models_to_check
File "/home/Robert/Desktop/odoo-12.0/odoo/modules/loading.py", line 222, in load_module_graph
load_data(cr, idref, mode, kind='data', package=package, report=report)
File "/home/Robert/Desktop/odoo-12.0/odoo/modules/loading.py", line 68, in load_data
tools.convert_file(cr, package.name, filename, idref, mode, noupdate, kind, report)
File "/home/Robert/Desktop/odoo-12.0/odoo/tools/convert.py", line 802, in convert_file
convert_xml_import(cr, module, fp, idref, mode, noupdate, report)
File "/home/Robert/Desktop/odoo-12.0/odoo/tools/convert.py", line 852, in convert_xml_import
relaxng.assert_(doc)
File "src/lxml/lxml.etree.pyx", line 3501, in lxml.etree._Validator.assert_ (src/lxml/lxml.etree.c:184715)
AssertionError: Document does not comply with schema