Why can I not just use the readings arbitrarily?
ok I agree I definitely left out a ton of details, I was just confused on why for instance 上 you needed to use one reading over another, but I have come to the conclusion that “it is how it is”
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Because you would be wrong
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You should think in terms of words, not readings. Specific words are just read in specific ways. 生きる is いきる, but 生える is はえる. Those are just the words that exist in the language.
Consider the following in English for comparison. “im” and “un” are both prefixes that mean “not”. You say “impossible”, but “unimportant”. Why? Those just happen to be the words that exist in the language.
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I agree it’s annoying but I’m getting used to it.
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This is like asking why can’t you just make up words on the spot and expect to be understood.
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Your question is too short, so it’s hard to know what you mean. Please explain more in detail what you want to know if you want good answers.
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The number “1” in English is “one,” but if you try to read “1st” as “onest,” you’re wrong, because that’s not a word.
This is basically how kanji are used and understood in Japanese; they can be read differently depending on whether they’re used alone, with other kanji, or with hiragana trailing afterwards. Many kanji are not even actual words if used by themselves (without other kanji/kana).
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That makes a ton of sense, thank you
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This is like saying why can’t I pronounce the English word “red” as “hippopotamus.” Japanese words are not just arbitrary mashups of readings like you’re playing a game of Boggle.
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Are you referring to the pink kanji reviews?
For those, the WK devs chose what they consider to be the “most useful” (most widely-applicable) reading for the kanji, so that if you encounter the kanji in a compound word you haven’t seen before, hopefully your first guess will be the correct reading. Most compound kanji words use on’yomi, so most of WK’s pink kanji reviews will ask you for on’yomi.
All that being said, which single reading is “most useful” is admittedly arbitrary, so if you input a correct reading that isn’t the one being asked for, you’re prompted for a retry instead of being marked wrong.
While we’re on the subject, I’d highly recommend that you install this userscript: [Userscript] WaniKani Katakana Madness
It’s a good way to passively keep straight which readings are on’yomi and which readings are kun’yomi, because the pink kanji reviews that want on’yomi will always display your answer in カタカナ.
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Unpossible! 
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Nah, it’s more like pronouncing “hippopotamus” as “river horse”. It’s technically a valid reading of the word, but everyone gonna stare at you and go “… what”?
It’s a perfectly cromulent word.
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No it’s not… 
It would always be “umpossible”.
did i miss the reference lol
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Embiggens your vocabulary!
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WK is not perfect, for example something like 額 could have ひたい (forehead) on the vocab entry but it’s not included and they limit it to one entry meaning for this one. It’s on the kanji reading page but not really a distinction between reading and meaning of which one goes with what. You can make suggestions but I find it useful to have a broad scope of the meaning through other means like dictionaries.
There an un a imimportant term.
You’ve never seen The Simpsons?!! 
I have, in fact, never seen The Simpsons lol.
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