未遂 is placed after the action it modifies: 谋杀未遂 (attempted murder), 政变未遂 (failed coup), 自杀未遂 (attempted suicide). It functions as a post-modifier in formal/legal language.
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未遂 is formal and most commonly used in legal, news, or written contexts. In casual speech, people would say 没成功 (méi chénggōng) instead.