As the song says, "A spoonfull of sugar makes the medicine go down." There was also a singing group called "The Loving Spoonfull" at one time. Americans do say "sugar spoon" for a spoon placed in a sugar bowl for scooping the sugar. In the sentence under discussion we would most likely say "spoonfull of sugar" rather than "spoon of sugar." Recipes call for a "teaspoon of sugar" or a "tablespoon of sugar" but a "spoon of sugar" sounds a little strange.
Well, it's a spoon meant to scoop up sugar. And Duo is looking for amount, not the thing.
One spoon of sugar is enough. Accepted. Makes more sense to me depending on context. Context that we do not have so we imagine it.