。
I need a nut that fits that bolt.
Literal
That bolt [to-に] fitting nut [subject-が] necessary [copula-です].
A technical/hardware sentence. ボルト ('bolt') and ナット ('nut') are loanwords for fastener components. 嵌合する (かんごう/はめあいする, 'to fit together') is the technical term for mechanical mating of parts — much more precise than just 合う. The relative clause そのボルトに嵌合する modifies ナット. 必要です ('is needed') is the predicate. Reads like a hardware request or engineering specification.