The expense could not be synced because it has no effect. In other words:
there are no items (for an expense claim or cash report), or
there are no legs (for a mileage claim), or
the expense items are for £0.00, or
the transaction amount is £0.00 (this could happen if it was a card check transaction)
It is safe to ignore this error and there is no need to retry synchronising the item. If the expense is changed to have an effect it will be synchronised immediately.