I can’t speak to the app, but it sounds like you’ve only unlocked kanji so far. Kanji will always test you on a specific reading (on’yomi or kun’yomi). Once you start learning vocabulary, you’ll learn additional readings.
For example, for the kanji 音 you’ll learn the on’yomi おん, which is very common. Once you level that up to guru, you’ll unlock vocab using that kanji. This will include the word 音 itself, which is read おと. Note that not every kanji can be read as a word on its own, but many can be.
At some point you’ll learn vocabulary that uses this kanji and you’ll be taught to use one or the other (or the kun’yomi). It is good to pay attention to the multiple on’yomi, but you’ll have to use the other one eventually, even if you only focus on one for now.
There are multiple vocab words with 力, you can check the list on
(scroll down to “Found In Vocabulary” section).
In particular, horse power 馬力 uses りき reading of the kanji. And the other reading would be wrong for this particular vocab.
I don’t use the app, but I saw the post recently about it being too lenient and prompting user several times for vocab reading, which is not the plain WaniKani experience.