When Jukai Preserver enters the battlefield, put a +1/+1 counter on target creature you control.
Channel — {2}{G}, Discard Jukai Preserver: Put a +1/+1 counter on each of up to two target creatures you control.
Discarding the card is part of the cost to activate a channel ability.
If a channel ability requires a target, you may not activate it without a target just to discard the card.