"Don't approach an angry parrot."
Translation:Noli appropinquare psittaco irato.
As another comment states, you misspelled appropinquare.
However, I also think that you are not supposed to separate the phrasenoli appropinquare, i.e., I think that putting psittaco irato in between the two words is incorrect. Although, Duolingo seems rather lax overall regarding word order in this Latin course and I could be wrong so maybe your answer should be accepted?
The word noli here is the singular imperative form of the verb nōlō, nolle, meaning "to be unwilling". (It is the negated version of vōlō, which is "to be willing, to want".) Hence the literal translation of noli appropinquare is the command "be unwilling to approach". As a result, I don't think it makes sense to separate noli and appropinquare, though I'm not an expert and I could be wrong.
I think they try to use the most common word order for the phrases and sentences, so that people doing the course will learn a more natural Latin than if the word order was random every time.
And for translation into Latin they have to add all the possible word orders themselves.
I tried two different answers:
Psittaco irato nolite appropinquare
Psittaco irato noli appropinquare
I've checked these against the books I have and these both appear to be correct for saying "do not approach the angry parrot", one for 2sg, one for 2pl.
DL even gives what looks like the same answer: "Noli appropinquare psittaco irato".
But both of my answers were marked wrong.
Is there a specific rule I've missed that says nolo has to be at the start of the sentence, or is this a case of DL not knowing this a correct variant?