The problem is with 口. It used to accept こう as a reading, but now it won’t. The same problem is happening with 力.
How am I supposed to know whether they want me to answer with onyomi or kunyomi?
You have to pay attention to the colors…
You learn pink first then purple.
I remember こう is pink and くち is purple.
Also りょく is pink and ちから is purple.
Pink is usually on-yomi. Vocab with hiragana attached is usually kun-yomi, but everything has exception.
Vocab (purple) has the one and only one reading.
Kanji (pink) has multiple readings, which may or may not be the same as the purple.
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As others have mentioned, kanji can have different readings whereas the vocabulary (purple background) has a single reading. Just thinking of it in terms of colors is okay, but it is best to think of it as vocab/kanji.
ETA: for the kanji if you enter the wrong reading usually it won’t mark it as wrong, it’ll just “shake” and tell you it’s looking for the other.
The pink background means it’s a kanji, the purple one means it’s vocabulary. Whilst a kanji can have and usually has different readings, a word has just one reading (there are exceptions even for this).
Most words consist of two or more kanji joint together (compound words) or a kanji with hiragana, but there are also words that consist of only one kanji and nothing more. These words usually use the kun’yomi reading, for your examples that would be くち for 口 (mouth) and ちから for 力 (force, power). Those are actual words that you can use in sentences, whilst you can’t say こう for 口 as the word ‘mouth’.
Yet, if you create compound words like 水力 (すいりょく - water power), those usually use the on’yomi reading.
Of course, here again there are exceptions, like 出口 (でぐち - exit).
So don’t mix those two up, one is the kanji and has various readings (and you need to type in the one(s) that is/are learnt in the lesson) and the other is the vocabulary and uses just one reading, just like we only use one reading for our words too. 
Wow! Some much help in such a short time. Thanks. I think I’ve got it.
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