Just FYI, that’s because you did not originate this topic (it was started by @minh5). Only the person who started the topic can mark a solution — which makes sense, since the solution to your apparently similar question might not have actually worked for the topic originator.
This is one reason why in general, it’s probably a better idea to post a new linked topic, rather than to jump in to an old thread with a similar problem. The forum software explicitly supports topics derived from other topics (though I wish it were more obvious!). You start by clicking the little link button at the bottom of the original post: