No Errata
Q: The Necromancer Commands allows a fighter, having failed an Attack action, to ‘make the Attack action again’. How does that interact with an Attack action that targets more than one enemy, such as the Harvester’s Whirling Scythe?
A: An attack that targets more than one enemy is treated as a number of individual Attack actions performed one after the other, and each of those Attack actions is judged as succeeding or failing based on whether or not its target is damaged. If one of the Attack actions made as part of the Whirling Scythe Attack action fails, you can play The Necromancer Commands to make that single Attack action targeting the same fighter again.
Q: Could Last Chance and The Necromancer Commands be triggered by the same Attack action?
A: Yes. Last Chance is a reaction taken during an Attack action that lets you ignore the damage caused by the Attack action. Only Attack actions that cause damage are successful, so the Attack action fails. At that point, if the attacking fighter was one of the Sepulchral Guard, their player could play The Necromancer Commands, which is a reaction taken after an Attack action that fails.