Element "completion" is satisfied solely as a logical result of the completion of ALL constituent checklist tasks within the element.
If you add a due date to an element that has no existing checklist tasks, the app will automatically add an "Element complete" checklist task to the element. This auto-added checklist task can now be edited in the usual way (the principal rule is that an element requires at least one checklist task to be "completable" i.e. for an element "due date" to have meaning).
Once all constituent checklist tasks within an element are ticked, the element will then be marked as complete in all relevant dashboard reporting tables.