ExceptionActiviti: unknown property used in expres

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.