Hi Lee,
That’s a missing routeing, sadly. It is available for Woodgrange Park (the station before Barking) so I think it’s an error. You’ll need to use either the Camden Town/Road or Kentish Town/West changes and touch the pink reader at Stratford to get the £1.80 off-peak fare. You could also email oysterenquiries(at)tfl.gov.uk and ask them to add the missing route, but don’t try to use it until it appears on the fare finder.