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"...

I'm pretty sure this could also be plural: "Please open the doors".

"Please open the gate" should also be acceptable

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.

Can "Please open up" be accepted?

Well im surprise that dont allow please open door