My friend and I can’t agree upon a ruling. I played my Kraul Harpooner, which has undergrowth and can target a flying creature against his Order of Midnight, which can’t block. Is Midnight a legal target for Kraul?
Kraul Harpooner's triggered ability can target Order of Midnight regardless of whether it can "fight", whether it can deal damage, or whether damage it would deal has to be prevented (e.g., because it's enchanted by Muzzle). All that ability requires of the target, if the choice is made to target one, is that it must be a "creature with flying you don't control" (C.R. 115.1d). It doesn't matter what the ability would do to that target when it would resolve.
Since you probably were befuddled by the fact that Order of Midnight can't block, I will assume that some among your group have yet to learn the difference between 'Fight' and 'Combat', as WizardMN pointed out (a very understandable mistake, one might say).
Those are two very different and unrelated concepts, in Magic.
You might consider having a peek at the actual rules for combat (rule 506) and fight (rule 701.12).
EDIT (Feb. 24, 2021): Edited.
Those are two very different and unrelated concepts, in Magic.
You might consider having a peek at the actual rules for combat (rule 506) and fight (rule 701.12).
Rules of Magic:
https://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules
The Rules Team even went as far as adding an extraneous rule to ward off confusion:
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules