Follow the steps below in order to setup the Approval Validation on subtasks, parent of subtask, epic or issues in epic validator on a workflow transition: 

1. Choose   →  Issues>Workflows  or click Create Workflow function on the Advanced#Workflowfunctions administration screen
2. Edit the concerned workflow
3. Select the concerned workflow transition
4. Select the Validators tab
5. Add Validator
6. Check the option for the Approval Validation on subtasks, parent of subtask, epic or issues in epic
   
   
7. Fill the parameters: 

   1. User used to evaluate the applied criteria ^_Required:  Any of the specified criteria will be evaluated  basing on the selected user. Ensure that the selected user has appropriate browsing permission on the projects at whom the current and the related issues belong to.

   2. Issue type for related issues pending approvals ^Required: the check on passed Approval Conditions will be performed only on the issues related to the current issue via the specified criteria

   3. Message to show when validation fails: A custom message (optional) to show when the validator returns false. When it's not specified, a default message appears: You cannot transition this issue because the Approval Validator on related issues is not yet satisfied

   4. JQL filter for related issues: the Approval Conditions will be checked only on the issues that satisfy the JQL filter

   5. JQL filter for current issue: the validator will be checked only if the current issue satisfies the JQL filter.