In the heroic duel mode, some cards are not displayed(they are not banned). Example: Lady Liadrin and others cards.
You do have the cards you are missing in your collection?
Hearthstone has some game modes where you have access to all cards, even if you don’t own them. Duels is not one of those. In Duels, the cards you put in your starter deck need to be cards you possess in your collection.
(Cards you add later from choosing treasures and buckets are not thusly limited).