"The test is not difficult."
Japanese doesn't need an article for all the words and そのmeans "that". Thus it is perfectly normal to translate "the test" as テスト.Actually it is more natural in Japanese like this. Yet, duolingo said it is wrong...
This isn't that much of an issue, but if you type the answer in Japanese instead of using the word bank, it's marked wrong if むずかしい isn't written in kanji. I don't even recall learning that kanji before.