For this Challenge, players would need to get a Blackjack. This is earned by getting 21 with just two cards.
Types of card combinations for a Blackjack:
Ace + 10
Ace + Jack
Ace + Queen
Ace + King
Note: Any combination of three cards or more for a 21 is not a Blackjack.
When the hand is a Blackjack, the game will let you know, like in the screenshot below:
When the hand is a normal 21, the game will let you know this as well, like in the screenshot below: