Hey there. This is to do with how mods can be refunded to reallocate the purchase points, and is nothing to do with their function. If a mod can't be refunded, it will show that message. Example - you need three mods in column 1 to be able to buy a mod in column 2. If you have bought a column 2 mod and only have three mods unlocked in column 1, all three will say 'requirements not met' - none of them can be refunded because then you'd have too few column 1 mods. None of this affects how the mod effects work.