This is shown as a warning because Mobius is verifying the status of whether the invoice already exists in our system. The task list is designed to present the result in this sense. If the invoice does exist in the system, you will see a warning message next to the task. Not to worry, even though the task user interface itself will show this as a warning, the invoice itself was successfully batched over to QuickBooks Online.
From the task like you will be able to resend the invoice directly from this screen. Please see our How do I resend an invoice? for more information.