It is quite normal, that big pups/young dogs start to bark. They feel they are BIG boys/girls, but still insecure and still to immature to handle the situation.
I use to tell the “teenagers”: Ja, ja, ja, I heard you…go lay down!... if they bark in the middle of a night, when the wind shakes the trees outside, when some human teenagers have fun on the footpath Saturday night
or what ever they hear, that I don't hear!!!