"请开门。"

Translation: Please open the door.

[Andrew-Lin]

請開門。
Fun fact: In traditional Chinese, both open "開"(simp. 开) and close "關"(simp. 关) consist of a door "門(simp. 门)". That's how the characters are created.

[ToEach_TheirOwn]

I really wish duo would work on their hints for Mandarin. Saying that this means "please open" isn't really helpful the first time you're learning the word for "door"...

[volkei]

It almost sounds like "come in"