Wouldn't it be "Why can't I choose an avatar? Or why can I not choose an avatar?
Thanks though, this has helped me a lot.
Can't is a contraction of
cannot, so your first suggestion is technically exactly the same as the question, the only difference being that
Why cannot I choose an avatar? sounds a little unusual, despite being more formal. Both options you've provided, as well as the question itself, have identical meanings and are correct in everyday English. Of the three the current question is the most formally correct, as strictly speaking
Why can I not choose an avatar? means you have the option of displaying no avatar, and you're asking why you're given that option.
All that is of little relevance, however, when you consider that both the question and answer aren't that hard to understand, as well as being one of the main FAQs you might expect to see.
That said, welcome to the forums!