If you are talking about Automation for Jira addon, then I'd say you need two separate rules. The reason you are getting this error is most likely because the supposed parent issue does not exist yet when the rule is triggered (not run!).
When Automation for Jira rules are triggered they are placed in a sort of queue withing the database along will all related data (issue field values for example). Then the rules are executed from that queue.
With that in mind, what most likely happens in your case is that the rule was placed in the execution queue when the parent issue didn't exist and even though by the time the actual execution kicks in the parent may've already been created it will still be ignored, because the rule is executed using the data from the queue and not the actual data.
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