If you research the order in both PrestaShop and at Simplify.com you might see that the underlying transaction was approved. However, there is a PresaShop setting that controls what happens when transactions are submitted against back ordered or "out of stock" items.
You can access this setting via:
- Go to Orders then Statuses
- Edit the status causing the issue (BackOrder in this case)
- Enable the setting "Consider the associated order as validated"
That should cause PrestaShop to properly evaluate the transaction status received from Simplify. Completing the order if APPROVED.