I am looking for a suitable trigger to use that will check all sub-task status and then transition the parent status based on a result.
eg: If any subtask status = REJECT, transition parent status to FAILED
I tried a scheduling trigger but that resulted in an error;
Can't branch rule as it requires issue(s) in the context for this option. More than likely you have a trigger that doesn't insert issues into the rule chain. e.g. Scheduled rule not running a JQL query. Creating an issue does not put an issue into the context, you have to branch on it directly.
Outside of the above, I don't see a suitable trigger for a workflow based on a subtask event.