пОлки not полкИ, wrong stress again!
Why do we need the definite article in "the shelves"?
You don't - it should be accepted. I just reported it.
Why на полки? (genitive, right?) I put на полке (prepositional, common after на), and DL said that I had a typo.
Because it's the accusative case in the plural form. Putting something onto something indicates a direction, that's why we need the accusative case here.
and the pronounciation is пОлки, correct?
Yes but полкИ is another word in plural. Полк - Regiment
Sound is incorrect. Not полки' (regiments), по'лки (shelfs).
The "Поставь" in the audio sounds almost like "Постах" ...