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If you start complaining, there's no end to it.
Literal
Complaints [object-を] say-[if-ば] limit [subject-が] not-exist.
切りがない ('there's no end/limit') is a common idiom. 文句を言えば is the conditional of 文句を言う ('to complain'). The proverb-like quality comes from the conditional + inevitable consequence structure — 'if X, then inevitably Y.' A universally relatable observation about the futility of complaining.