if you have so much cards to disenchant probabily you have an old bug.
This bug …
caused this bug
So game was telling you to disenchat a card that is from your collection becouse you already have a free version from core set , but thats not true , becouse if core set changes you lose free core cards , so you need to keep your personal copy of that cards.
So game count this cards for disenchant , but in real game dont disenchant this cards with mass disenchant.