Because this Chinese course was in beta until recently. We're expected to suggest the missing answers. The Duolingo team then goes through and checks all our suggestions and implements the ones they confirm. This takes a varying amount of time.
In proper English, we SHUT doors and we don't "close" doors. I wrote "Don't shut the door" and got a failing grade. So I will have to slavishly accept "Don't close the door" if I want to proceed to the next module.
@RegWong - In proper English "A nod is as good as a wink to a blind horse" becomes "A slight inclination of the cranium is as adequate as the spasmodic movement of one optic to an equine quadruped utterly devoid of visionary capacity".. :-)