A: I am so tired.
B: Why is that? (I don't think it is natural to use "that" in this situation. "So tired" is difficult to quantify.)
The second question and answer pair is fine to me.
I want to buy some snow boots in the/a sale. (or "a pair of snow boots").
The next sentence is fine grammatically. It is difficult to "cure" a pimple, however. Perhaps "I want the pimple on my face to clear up as soon as possible."
"We can only update our notebook twice a day on italki." is my preference.