Can't use parent as a group by option when using custom Charts by JIra

[Nannette Mori]

Jira Version 8.20.1

Custom Charts for Jira 4.5.2

I am trying to use the Stack Bar Chart and group by parent for a subtask.  When I try to  group parent is not available in the group by option.  When I try to add Parent Link to my subtask I receive error message to add subtask link for parent.  I cannot locate the subtask link.  How do I access parent to have that field available in by group by Option in my chart???