In this example, moving the current issue to status "Done" should only be possible when all sub-tasks are in status category "Done" - otherwise the transition button should be hidden.
For this, a Boolean condition and validator with math. date-time or text-string terms is needed for the transition to "Done" on the current issue (the parent).
The configuration will look like this:
Boolean expression to be evaluated is: count(filterByPredicate(subtasks(), ^%{00144} != "Done")) = 0
Note that:
After adding the condition, the issue cannot be transitioned as long as there are sub-tasks that are not yet in status category "Done":
As soon as all sub-tasks are in status category "Done", the parent issue can be transitioned as well: