In this use case, we will prevent the creation of sub-tasks when the parent issue of the sub-tasks is in any status different from the one that we specify.
Add the Logical validator to the Create workflow transition.
Add the following expression replacing the name of the status with the status of your choice:
With this solution the user will get an error message when he clicks on Create after filling up the creation screen. A solution that prevents the user from filling up the sub-task’s creation screen is not possible.
Add an error message as the following one.