ExceptionActiviti: unknown property used in expression - workflow

Question

sI use activiti for my workflow on alfresco 4.2.c. I want to assign to a group from my form selection. But when I press submit task it throws an exception: org.activiti.engine.ActivitiException: Unknown property used in expression
My activiti is:
<userTask id="kiemtranoidungthethuc" name="Kiểm tra nội dung, thể thức VB- Chuyển cho phòng TCHC" activiti:assignee="${bpm_assignee.properties.userName}" activiti:formKey="wfdi:kiemtranoidungthethucTask_tc">
<extensionElements>
<activiti:taskListener event="create" class="org.alfresco.repo.workflow.activiti.tasklistener.ScriptTaskListener">
<activiti:field name="script">
<activiti:string>
if (typeof bpm_workflowDueDate != 'undefined') task.dueDate = bpm_workflowDueDate;
if (typeof bpm_workflowPriority != 'undefined') task.priority = bpm_workflowPriority;
</activiti:string>
</activiti:field>
</activiti:taskListener>
<activiti:taskListener event="complete" class="org.alfresco.repo.workflow.activiti.tasklistener.ScriptTaskListener">
<activiti:field name="script">
<activiti:string>
execution.setVariable('wfdi_userVanthu', person);
execution.setVariable('wfdi_chapthuan_tc', task.getVariable('wfdi_chapthuan_tc'));
<!-- execution.setVariable('bpm_assignee', person);-->
execution.setVariable('bpm_dueDate', task.getVariable('dueDate'));
execution.setVariable('bpm_priority', task.priority);
execution.setVariable('bpm_groupAssignee',task.bpm_groupAssignee);
</activiti:string>
</activiti:field>
</activiti:taskListener>
</extensionElements>
</userTask>
<userTask id="kiemtrathethucchuky" name="Phòng TCHC - Kiểm tra thể thức, chữ ký - Chuyển cho BGH"
activiti:candidateGroups="${bpm_groupAssignee.properties.authorityName}" activiti:formKey="wfdi:kiemtrathethucchukyTask_tc">
<extensionElements>
<activiti:taskListener event="create" class="org.alfresco.repo.workflow.activiti.tasklistener.ScriptTaskListener">
<activiti:field name="script">
<activiti:string>
if (typeof bpm_workflowDueDate != 'undefined') task.dueDate = bpm_workflowDueDate;
if (typeof bpm_workflowPriority != 'undefined') task.priority = bpm_priority;
<!-- if (typeof bpm_comment != 'undefined') task.setVariable('bpm_comment', bpm_comment);-->
</activiti:string>
</activiti:field>
</activiti:taskListener>
<activiti:taskListener event="complete" class="org.alfresco.repo.workflow.activiti.tasklistener.ScriptTaskListener">
<activiti:field name="script">
<activiti:string>
execution.setVariable('wfdi_kiemtra_tc', task.getVariable('wfdi_kiemtra_tc'));
execution.setVariable('bpm_dueDate', task.getVariable('dueDate'));
execution.setVariable('bpm_priority', task.priority);
</activiti:string>
</activiti:field>
</activiti:taskListener>
</extensionElements>
My form is:
<config evaluator="task-type" condition="wfdi:kiemtranoidungthethucTask_tc">
<forms>
<form>
<field-visibility>
<show id="message" />
<show id="taskOwner" />
<show id="bpm:priority" />
<show id="bpm:dueDate" />
<show id="bpm:taskId" />
<show id="packageItems" />
<show id="bpm:groupAssignee" />
<show id="bpm:comment" />
<show id="wfdi:chapthuan_tc" />
</field-visibility>
<appearance>
<set id="" appearance="title" label-id="workflow.set.task.info" />
<set id="info" appearance="" template="/org/alfresco/components/form/3-column-set.ftl" />
<set id="items" appearance="title" label-id="workflow.set.items" />
<set id="assignee" appearance="title" label-id="workflow.set.assignee" />
<set id="response" appearance="title" label-id="workflow.set.response" />
<field id="message">
<control template="/org/alfresco/components/form/controls/info.ftl" />
</field>
<field id="taskOwner" set="info" />
<field id="bpm:taskId" set="info">
<control template="/org/alfresco/components/form/controls/info.ftl" />
</field>
<field id="bpm:priority" set="info" read-only="true">
<control template="/org/alfresco/components/form/controls/workflow/priority.ftl" />
</field>
<field id="bpm:dueDate" set="info" label-id="workflow.field.due">
<control template="/org/alfresco/components/form/controls/info.ftl" />
</field>
<field id="packageItems" set="items" />
<field id="bpm:groupAssignee" label-id="workflow.field.review_group" set="assignee" />
<field id="bpm:comment" label-id="workflow.field.comment" set="response">
<control template="/org/alfresco/components/form/controls/textarea.ftl" />
</field>
<field id="wfdi:chapthuan_tc" label-id="workflow.field.outcome" set="response">
<control template="/org/alfresco/components/form/controls/workflow/activiti-transitions.ftl" />
</field>
</appearance>
</form>
</forms>
My model is:
<type name="wfdi:kiemtranoidungthethucTask_tc">
<parent>bpm:workflowTask</parent>
<properties>
<property name="wfdi:chapthuan_tc">
<type>d:text</type>
<default>Tu_Choi</default>
<constraints>
<constraint name="wfdi:chapthuanOption_tc" type="LIST">
<parameter name="allowedValues">
<list>
<value>Chuyen_Phong_TCHC</value>
<value>Tu_Choi</value>
</list>
</parameter>
</constraint>
</constraints>
</property>
</properties>
<overrides>
<property name="bpm:packageItemActionGroup">
<default>edit_package_item_actions</default>
</property>
<property name="bpm:outcomePropertyName">
<default>{http://www.alfresco.org/model/workflow/1.0}chapthuan</default>
</property>
</overrides>
<mandatory-aspects>
<aspect>bpm:groupAssignee</aspect>
</mandatory-aspects>
I know the problem is: activiti:candidateGroups="${bpm_groupAssignee.properties.authorityName}"
but I don't know how to fix it.
Can anybody help me?
Thank in advance.