someone says this in a book:
whether someone is skilled ... something to do with personality. what does ありき mean here? I found it as a noun meaning location of something but I don't see how it would work in this sentence
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First, the answer to your question is yes. You need to change the copula だ into the attributive form such as …である or …な when you connect the clause 人格ありきだ to the noun わけ.
When you combine two sentences of 人格ありきだ (one's personality is prior to it) and わけだ (it's for the reason) into one sentence "It's for the reason that one's personality is prior to it", you can't just put them in a row like 人格ありきだわけだ, but you have to change だ as in ありきだ into some forms that can connect to the succeeding noun, which are である or な.
In this regard, verb ある happens to be the same form either when it terminates the sentence or when it connects to a noun. However, だ only works when it terminates the sentence and な only does when it connects to a noun.
Second, Aは Bありき だ means that B is a precondition or an essential condition for A. So, the example sentence means "But, whether you can activate your skill or not depends on your personality, so...".