If you have a soft 21 (a hand made of 3 or more cards or a hand made of 2 cards following a split totaling 21 with an ace acting as an 11) or natural, yes, you can hit. You cannot hit a hard 21.
If you hit or double your natural, you are forfeiting the 3:2 payout (and decreasing your chances of winning). So, it would never be a good move.
Hitting a soft 21 is likewise unhelpful. However, there are technically times when doubling your soft 21 or natural increases your EV. If the remaining cards are good enough (such as because the count is high), then it is worth it to reduce your chance of winning for the 2:1 payout even sometimes with a natural where you give up the guaranteed 3:2 payout.
If you hit and go over 21, you of course automatically lose.