'πρόκειται να διαβάσουμε το βιβλίο' - it's clear than "we" without 'εμείς'. Why duolingo punishes me for not stating 'εμείς' ?
There is clearly some error. We have exactly what you say: "Πρόκειται να διαβάσουμε το βιβλίο." as the best answer. Please tell us where you found this sentence. Was it on the Tree, Strengthen skills or Test out. It will help us a lot.
I saw the same error on web, on the tree, while learning.
I had that same problem, but then I realized that I spelled διαβάσουμε wrong. I didn't know that there is a sigma instead of a zeta in the subjunctive.
The Greek Forum has two stickies with really good explanations for the subjunctive. https://forum.duolingo.com/topic/936
Εμεις πρόκειται να διαβάσουμε το βιβλίο.
We are going to read the book.
Εμεις πρόκειται να διαβαζουμε το βιβλίο.
We are going to read the book (forever).