Hey everyone,
Our Hearthstone team is aware of this issue affecting the Duels mode and they are working to get it resolved. It is caused by copying a deck code in during Duels deck creation.
For anyone who is currently stuck in a Duels run with this error, please try queuing up again after Forged in the Barrens goes live on March 30th. The Hearthstone team will be closely monitoring the situation and making adjustments as needed.
Thank you all for your patience and the reports so far.