"Chuu" kanji from "Chuugoku" read as "Naka" in isolation
Throughout Intro 1 the kanji for "chuu" is pronounced as "naka" by the audio when selected, and while it didn't cause me much problem as I have some experience with Japanese clicking a character appearing for the first time and hearing "naka" and being expected to match it with hiragana for "chuu" made even me question my sanity for a minute before realizing it was an issue with the course
It is an issue with the course in the sense that it is not addressed at all! Like many others this also tripped me up the first time.
Truth is both "naka" and "chuu" are readings for that Kanji! In fact, pretty much every kanji (as far as I know) has multiple readings, aka pronunciation. There are two types of readings called Kun-yomi and On-yomi, where kun-yomi is the japanese-based pronunciation of the kanji and on-yomi is loosely based off of the chinese pronunciation of the same hanzi (chinese word for chinese character). Unfortunately there doesn't seem to be a telltale sign for which way to pronounce a kanji by looking at it since you have to know it based on the context of the word it is contained in.
Check out the online kanji dictionary Jisho for this character, and you should see that it indeed does have multiple readings (see the side bar). In the word for China it appropriately has the On-yomi reading in the word 中国, since both kanji are based off the chinese reading for China.
For more about kanji, Tae Kim's Japanese Guide is a nice resource.
Hope this helped a little, good luck!
I know you said "pretty much", but it's not extremely uncommon within 常用漢字 to find some with only one. Admittedly, most of these are less commonly used and/or in combinations, but OP deserves some hope at least. 峠「とうげ」, meaning "mountain pass" and 笹「ささ」, meaning "bamboo grass", for example. Frankly, neither of which I've used more than a handful of times, but useful to recognize. By the way, I wholeheartedly second the use of Jisho.
When a kanji is part of a larger word (combined with other kanji) it will take it's chinese-based On-yomi reading, which is "Chuu". But when a kanji is by itself it is read with its Kun-yomi reading which is "Naka". There are also multiples of each kind of reading depending on the context a kanji is used in.
Duo's reading of the isolated kanji isn't exactly wrong, by itself that kanji would be naka, but the system fails to account for the context of it and how the surrounding kanji change how it is pronounced.
I guess it could be similar to seeing the english word "through" pronounced like 'thru' as a whole, and trying to highlight the 'rough' part expecting to hear 'ru' but instead hearing how 'rough' as its own word is pronounced as 'ruff'.