preorder cars and 20th barn find

I keep getting marketing error when I go to click on the new pre-order cars like the 2016 Audi to v10 plus, 2015 dodge challenger srt hell cat, 2015 Ferrari too gtb, 2016 Ford Shelby gt 350r, 2016 jaguar f-type project 7 all pre-order cars but I can get in the Mercedes-amg c 63 s coupé. Now with the barn finds its saying 19/20 barn finds found. The newest one i found was the amc pacer. How do I unlock the 20th one

https://forums.forzamotorsport.net/turn10_postst94760_Pre-Order-cars-received-and-unusable.aspx

Unless you individually redeemed codes for them you cant access them…they were sent to a lot of people by mistake

The barn find is in an upcoming forzathon event just like the pacer was