From the manual, this is what the problem is about —
Anki starts with a deck called “default”; any cards which have somehow become separated from other decks will go here. Anki will hide the default deck if it contains no cards and you have added other decks. Alternatively, you may rename this deck and use it for other cards.
Default deck stays hidden when no cards are in that deck and you have other decks in Anki.
From Google search —