Error when create multi Subtask for multi user in a Group (get value from a custom field "Department

[VTB SAM4]

I create a Script to auto create Muti Sub Task for a Group User (Group User is got value from a custom field name "Department". However, I got error at this line code:

def groupsubtask = groupManager.getUsersInGroup(groupParent)

[Static type checking] - cannot find matching method
com.atlassian.jira.security.groups.GroupManager#getUsersInGroup (java.lang.ob)
Please check if the declared type is right and if the method exits.
@ line 24, column 20.

Here is my Script

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.crowd.embedded.api.User
import com.atlassian.jira.ComponentManager

def constantManager = ComponentAccessor.getConstantsManager()
def user = ComponentAccessor.getJiraAuthenticationContext().getUser()
def issueFactory = ComponentAccessor.getIssueFactory()
def subTaskManager = ComponentAccessor.getSubTaskManager()
def issueManager = ComponentAccessor.getIssueManager()

Issue parentIssue = issue

def customFieldManager = ComponentAccessor.getCustomFieldManager()

def DutyTypeCF = customFieldManager.getCustomFieldObjectByName("Duty Type")
def ParrentDutyType=parentIssue.getCustomFieldValue(DutyTypeCF)
def Subtaskcf = customFieldManager.getCustomFieldObjectByName("Duty Type")
def groupManager = ComponentAccessor.getGroupManager()
def groupcf=customFieldManager.getCustomFieldObjectByName("Department")
def groupParent= parentIssue.getCustomFieldValue(groupcf)
def groupsubtask = groupManager.getUsersInGroup(groupParent)

//def group = groupManager.getUsersInGroup(issue.getCustomFieldValue(groupcf))

groupsubtask.each { groupuser ->
MutableIssue newSubTask = issueFactory.getIssue()
newSubTask.setSummary("Schedule Test")
newSubTask.setParentObject(parentIssue)
newSubTask.setProjectObject(parentIssue.getProjectObject())
newSubTask.setAssignee(groupuser)
newSubTask.setIssueTypeId(constantManager.getAllIssueTypeObjects().find{
it.getName() == "Schedule"
}.id)

newSubTask.setCustomFieldValue(Subtaskcf, ParrentDutyType)
// Add any other fields you want for the newly created sub task

Map<String,Object> newIssueParams = ["issue" : newSubTask] as Map<String,Object>

// for JIRA v7.*
issueManager.createIssueObject(user, newIssueParams)
subTaskManager.createSubTaskIssueLink(parentIssue, newSubTask, user)

log.info "Issue with summary ${newSubTask.summary} created"
}

Very urgent...Please help.

Thanks so much!