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The main thing dreams lack is consistency.
Literal
Dream [to-に] lack [progressive-ている] main thing [topic-は] coherence [copula-である].
欠ける ('to lack, to be missing') takes に for the thing that lacks — 'to be missing in X.' 首尾一貫性 ('consistency, coherence') is a 四字熟語-based abstract noun: 首 (head) + 尾 (tail) + 一 (one) + 貫 (pierce through) + 性 (nature) = 'the nature of piercing through from head to tail' = 'consistency.' Dense formal writing.