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As before, if the code isn't even a Steam code, then Steam will say it's not valid because it really doesn't know the code, like, at all. It can't possibly tell you something like "Oh, this is a key for the xxx platform, use it there!". You'll have to use it on whatever platform it is for, and it's up to you to know.
Or you might have mistyped if, if it's a Steam key but you didn't get it in electronic form (I haven't typed a Steam-key in years, since I can just copy&paste them).
But in the same vein that you can't buy a game on Origin and also get a license on Steam for that purchase. Buying a game on physical disc doesn't (necessarily) get you a license on a random digital platform. Unless things are specifically configured for that game to allow it, you play off the disk or buy a new license in the format or on the platform you really want.
https://store.steampowered.com/app/254840/Ground_Control_II_Operation_Exodus/
but are you sure it's a Steam key you have? It may just be a CD code for installation or something. Where did you obtain this key and is it from a physical version?