Issue:

We can't update the due date using transition in the post function.

Suggestion

Allow us to update the due date(system field) during transition in the workflow as we are allowed to do for custom fields.

Workaround

1. Begin by creating a custom field named "Temporary Date".
2. Proceed to update this field using the same method you used for the start date.

3. Finally, utilize the Copy Value From Other Field feature to transfer the value from the "Temporary Date" field to the "Due Date" field.