How can i fix it. 

Traceback (most recent call last):
  File "/odoo13/odoo-server/odoo/http.py", line 619, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/odoo13/odoo-server/odoo/http.py", line 309, in _handle_exception
    raise pycompat.reraise(type(exception), exception, sys.exc_info()[2])
  File "/odoo13/odoo-server/odoo/tools/pycompat.py", line 14, in reraise
    raise value
  File "/odoo13/odoo-server/odoo/http.py", line 664, in dispatch
    result = self._call_function(**self.params)
  File "/odoo13/odoo-server/odoo/http.py", line 345, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/odoo13/odoo-server/odoo/service/model.py", line 93, in wrapper
    return f(dbname, *args, **kwargs)
  File "/odoo13/odoo-server/odoo/http.py", line 338, in checked_call
    result = self.endpoint(*a, **kw)
  File "/odoo13/odoo-server/odoo/http.py", line 909, in __call__
    return self.method(*args, **kw)
  File "/odoo13/odoo-server/odoo/http.py", line 510, in response_wrap
    response = f(*args, **kw)
  File "/odoo13/odoo-server/addons/web/controllers/main.py", line 1323, in call_button
    action = self._call_kw(model, method, args, kwargs)
  File "/odoo13/odoo-server/addons/web/controllers/main.py", line 1311, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "/odoo13/odoo-server/odoo/api.py", line 395, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "/odoo13/odoo-server/odoo/api.py", line 382, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "<decorator-gen-58>", line 2, in button_immediate_install
  File "/odoo13/odoo-server/odoo/addons/base/models/ir_module.py", line 72, in check_and_log
    return method(self, *args, **kwargs)
  File "/odoo13/odoo-server/odoo/addons/base/models/ir_module.py", line 463, in button_immediate_install
    return self._button_immediate_function(type(self).button_install)
  File "/odoo13/odoo-server/odoo/addons/base/models/ir_module.py", line 573, in _button_immediate_function
    modules.registry.Registry.new(self._cr.dbname, update_module=True)
  File "/odoo13/odoo-server/odoo/modules/registry.py", line 85, in new
    odoo.modules.load_modules(registry._db, force_demo, status, update_module)
  File "/odoo13/odoo-server/odoo/modules/loading.py", line 420, in load_modules
    loaded_modules, update_module, models_to_check)
  File "/odoo13/odoo-server/odoo/modules/loading.py", line 312, in load_marked_modules
    perform_checks=perform_checks, models_to_check=models_to_check
  File "/odoo13/odoo-server/odoo/modules/loading.py", line 222, in load_module_graph
    load_data(cr, idref, mode, kind='data', package=package, report=report)
  File "/odoo13/odoo-server/odoo/modules/loading.py", line 68, in load_data
    tools.convert_file(cr, package.name, filename, idref, mode, noupdate, kind, report)
  File "/odoo13/odoo-server/odoo/tools/convert.py", line 737, in convert_file
    convert_xml_import(cr, module, fp, idref, mode, noupdate, report)
  File "/odoo13/odoo-server/odoo/tools/convert.py", line 804, in convert_xml_import
    obj.parse(doc.getroot())
  File "/odoo13/odoo-server/odoo/tools/convert.py", line 722, in parse
    exc_info[2]
  File "/odoo13/odoo-server/odoo/tools/pycompat.py", line 13, in reraise
    raise value.with_traceback(tb)
  File "/odoo13/odoo-server/odoo/tools/convert.py", line 713, in parse
    self._tag_root(de)
  File "/odoo13/odoo-server/odoo/tools/convert.py", line 675, in _tag_root
    f(rec)
  File "/odoo13/odoo-server/odoo/tools/convert.py", line 578, in _tag_record
    record = model._load_records([data], self.mode == 'update')
  File "/odoo13/odoo-server/odoo/models.py", line 4033, in _load_records
    data['record']._load_records_write(data['values'])
  File "/odoo13/odoo-server/odoo/models.py", line 3971, in _load_records_write
    self.write(values)
  File "/odoo13/odoo-server/addons/point_of_sale/models/pos_config.py", line 452, in write
    raise UserError(_('Unable to modify this PoS Configuration because there is an open PoS Session based on it.'))
odoo.tools.convert.ParseError: "Unable to modify this PoS Configuration because there is an open PoS Session based on it.
" while parsing /odoo13/odoo-server/custom/pos_multi_session/data/pos_multi_session_data.xml:5, near
<odoo>

    <record id="default_multi_session" model="pos.multi_session">
        <field name="name">Default Multi Session</field>
        <field name="multi_session_active">False</field>
    </record>

    <record id="point_of_sale.pos_config_main" model="pos.config">
        <field name="multi_session_id" ref="default_multi_session"/>
    </record>

    <!-- Default multi session for exist POSes -->
    <function model="pos.multi_session" name="action_set_default_multi_session" eval="[[ref('default_multi_session')]]"/>

</odoo>