It's fine. "Main" is a noun. com/dictionary/american/main_2
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Mr. TomIs it OK to use underlined as a noun?Yes. Where it makes sense, adjectives preceded by a determiner can be used as nouns.
Mr. TomIs the underlined natural English?
Mr. TomPS: Is my use of underlined as a noun OK?It doesn't seem very natural to this speaker of BrE. I'd say: