My young Sunset apple tree (planted two years ago) had very little fruit this year although a reasonable amount of blossom. Only a few apples developed but dropped when about the size of walnuts. They were as hard as bricks, green and knobbly on the outside but brown inside... What could be wrong and how to prevent next year please?
There can be several reasons for fruit drop.
This article will help you.
https://www.gardeningknowhow.com/edible/fruits/apples/apple-trees-dropping-fruit.htm