So to start, I saw at the beginning of the challenge you was Extending lines in the wrong direction, but by moving forward in the exercise I noticed that you had corrected this. Also, you were extending only one vanishing point at the beginning but you have corrected it. However, I saw that often your lines are diverging or are parallel so watch out!

Here is a link for that: https://imgur.com/mWLlnYl

To continue I noticed that sometimes you were messing with the face that was facing to you, be careful with that. As well as your hatching lines often goes beyond the face, be meticulous with them. I will mention that often you are adding lineweight at a place where it's useless and they are also wobbly. You should also get a little bit more of variety in your box.

And to conclude a last note is Inner corner, a way to improve them is to start thinking about the relationships between lines instead of just thinking about the lines in pairs. When drawing the lines of the boxes, the student should keep in mind all the other lines in the set, comparing their angles and making a guess based on them. This diagram explain this more clearly:

https://i.imgur.com/8PqQLE0.png

The diagram can be pretty hard to understand at first, so if you don't understand it, don't get frustrated, keep reading it from time to time while practicing regularly, and it will click eventually.