Can't Use Hearthstone Deck

A Hearthstone deck will be invalid in the following situations:

• When one or more of the cards in your deck were disenchanted
• When your deck contained cards from the Core Set that have rotated with the new Hearthstone year 
• In Standard mode, when one or more of the cards in your deck moved to Wild 
• When importing from a deck builder website via a copy/paste and the code includes invalid cards for the mode

In the cases above, you must manually replace the invalid cards for others in your collection. If the missing cards were disenchanted, crafting them again will also resolve this problem.

You may also receive this error when trying to use a deck that is intended for a different mode. You can convert your deck through the deck builder by clicking the buttons "Conver to Wild" or "Convert to Twists". 

Finally, if you are receiving this error during an active Arena and Brawl run, the most likely reason is that your deck became "illegal" due to a new game patch. In this case you will need to retire from your active run and your Tavern Ticket will be reimbursed.