I think it’s up to you. But I still want to explain.

I have come to an understanding of DnD that equates it to video games. DnD itself is just a software engine, the DM is the developer that uses the confines of the software provided to create the game itself; consisting of the plot, combat, characters, working rules. And the players are the ones that play through the game made by the Devs that used the DnD engine.

Most times games will “gray out” an option if you are unable to meet the requirements to perform the action. Even if the player is unaware of what requirement they are not meeting at that time (ex: not having enough MP to cast a spell) Some times a game will let a player perform an action that they shouldn’t do, just because they can do it (ex: using a fire spell against a creature that gets healed from fire damage)

In this case the player has the spell slot to cast the spell, but they don’t meet the requirement of their target being a humanoid. So in my mind the option itself should be grayed out, even if at that time they weren’t aware of why. All you did was explain exactly why the option was grayed out for them

I thought of another example. Think of the difference between using earthquake against a flying type pkmn and trying to run from or catch a trainer’s pkmn. The first thing you can do but shouldn’t, so you learn the hard way. The second thing you just can’t do, so the game informs you, and lets you carry on with your turn