Whenever i try to queue as deathknight it says ''There was an error starting your game. Please wait a few minutes and try again. I’ve only restarted playing hearthstone a few days ago and recently unlocked the deathknight. I even tried using the premade deck ‘‘Plaguebringer’’ in the solo modes and it says I can’t use that deck, but the other premade decks work fine.
The best thing to try in this case is to delete the deck and then create it again, not by using a deck code but by manually putting the cards in. (So do save a screenshot of the decklist first).
The technical explanation: some cards have multiple versions, and a copied deck code might contain a version of a card that is no longer valid. By manually creating the deck, you can only put valid versions of cards in.