Since you’re the author of the post, you’re the only who would be seeing that button, I’m afraid. I tried to look in some of my old posts, but they either don’t have any replies I can choose as solution, or they already have one.
Lately I’ve been experiencing lots of issues with GitHub’s WebUI, here and on GitHub too, including missing previews of the markdown text being typed (here on Community), and missing buttons or UI elements, and very often broken CSS. Usually, refreshing the page multiple times solves the problems, but in this case I suspect it might be a backend bug which for some reason is not treating this thread (or category) as one that allows “solution answers”.
Well never mind. The goal of marking a solution is to allow spotting solved posts in posts-lists, and having the solution answer always show on top. But that’s not a very long thread either.