The invoice is missing invoice date in the coding, it's possible that someone changed the field to make it non-mandatory.
Error in the sync tool
The invoice will be in sync error status on the Export tab, Reset the invoice and add the date to the invoice and resubmit for approval again.
To prevent this from happening again the System Admin can change the settings to make this field mandatory when submitting invoices for approval.
We advise that the System Admin also check that other settings haven't been changed.