Inaptness is the noun: the quality of not being the right fit, especially of a phrase, a headline, or a remark. It names the mismatch, not a person’s lack of skill. You will meet it in careful writing more than in talk; in everyday English, “not the right word” or “a poor fit” will sound more natural. Though actually, when writers do choose it, they often want that small, exact idea: the line was not stupid, and the speaker was not clumsy — the wording just did not belong. That is inaptness. The cousin ineptness would point at clumsiness instead.