"He reducido la cantidad de azúcar."

[y.ademas]

"I have reduced the quantity of sugar" should also be accepted.

[roselaw]

Agreed. I just reported it. Did you?

[Ma.Teresa202827]

this a mistake made by duolingo, he has reduce the amount of sugar is correct.

[marcy65brown]

He has reduced the amount of sugar = (Él) ha reducido la cantidad de azúcar.

[AdanRgz]

Puedo usar en este caso quantity en vez de amount?

[meryidrobo]

the sentence says he that is the third person in singular for this reazon I write has

[roselaw]

"He reducido" means "I have reduced." First person singular, not third.

[figueneda]

Por qué no es valido: "I have reduced the sugar amount"

Doy dos lingotes al que me explique donde está mi error.

[roselaw]

LOL, it's one of those things that native English speakers know, without being able to explain why. You can say "I have reduced the amount" but as soon as you say what it's an amount of, you have to say "I have reduced the amount of [whatever]. It just is what it is.