Hi @simplyjojo76
The worklow is failing because in some cases the due date column in the planner taks is empty or null. In order to avoid that, you could use the following expression:
coalesce(items('apply_to_each')?['dueDateTime'],utcNow())
Coalesce function returns the first non-null value (so, if dueDateTime is null, current date time (utcNow) is returned).
Hope it helps!
Ferran