How to make a cascading field second level required only if second level there.

Teja, for your case where items might be childless... When the HashMap only has only selected item, you can pass its ID to OptionsManager.findByParentId(Long).  If that's not empty, it means the item has children that the user should have selected so you can make your validator fail.

How to make a cascading field second level required only if second level there.

A

A1, A2

B

B1, B2

C

No second level

I tried to but is not working somehow.
no error and the transition pass.
I tried to have a simple script-runner validator and it works.
The code is : cfValues[}}{{"The name of your customfield"}}{{]?.keySet()?.size() == 2
https://scriptrunner.adaptavist.com/4.3.4/jira/recipes/workflow/validators/simple-scripted-validators.html#_validating_cascading_selects 

Try using this slight variation:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue

def issue = issue as MutableIssue
def cascadingSelect = ComponentAccessor.customFieldManager.getCustomFieldObjects(issue)?.findByName("Cascading Field Name")
def values = issue.getCustomFieldValue(cascadingSelect) as HashMap

// Will return true if both the parent and child options in the Cascading Select have a value
def keySetSize = values?.keySet()?.size()
keySetSize == 2

I have added the last link in the workflow in the creation field and somehow the code is not working for me.
The script is not showing any error in the logs but the transition pass

I used the code posted by @David Lawless, and it worked fine. No errors. Just input your custom field name here: findByName("CascadingSelect")

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue

def issue = issue as MutableIssue
def cascadingSelect = ComponentAccessor.customFieldManager.getCustomFieldObjects(issue)?.findByName("CascadingSelect")
def values = issue.getCustomFieldValue(cascadingSelect) as HashMap

// Will return true if both the parent and child options in the Cascading Select have a value
values?.keySet()?.size() == 2