I might be wrong, but if きたない is an adjective, then why isn't the translation "not dirty" or something similar since a "ない" conjugation of an adjective makes it negative? Also, is きたない a "な" adjective or an "い" adjective? I hope this made sense, I might be wrong since I haven't been in a Japanese class in a while and I'm in need of practice.
Don't trust google translate. It is terrible at distinguishing the context a word is being used in and will only provide you with a single answer, often the incorrect one. Stick with an actual dictionary like Jisho.org which will provide you with all possible translations and information on how each one is used in context.
汚れた・けがれた is the past tense of the verb 汚れる・けがれる to be violated/polluted/corrupted
It can also be pronounced よごれた for "to become dirty/get dirty"
汚い・きたない is an adjective meaning "dirty"
ない is a negative ending, however despite how the hiragana looks the kanji for dirty is 汚い. So it's read Kitana-i, not Kita-nai.
The negative of 汚い is 汚くない. Kitana-kunai.
Kita-nai (with different kanji) is actually informal past-negative of the verb "to come", so "Did not come"
Are you saying you wrote your answer in romaji, the writing system used for teaching foreigners who can't read Japanese how to pronounce words, instead of in hiragana, the Japanese script? Even though the entire purpose of these skills is to teach you hiragana...
Duo does not accept romaji answers. You can either use the word bank or install a Japanese IME on your device to type the answer in yourself.