The only person who is able to accept an answer is the person who asked it. Moderators don't have the power to accept answers on anyones' behalf (nor to unaccept an answer).
Moderators aren't expected to be experts in everything that gets posted on the site. We're here to support the community, handle flags and deal with urgent issues. Whether or not an answer is accepted isn't something that requires intervention - answers get votes and the ones with the highest number of votes should generally mean it's a correct useful answer, so people reading the site have that to go by.
As a community site it's really up to the users involved if they want to accept an answer or not. It's encouraged though, that's why there is a rep reward for accepting an answer, but it's not really a problem if answers aren't accepted. It's the votes that count. Sometimes an answer gets accepted that isn't the highest voted answer, sometimes blatently wrong answers get accepted. But this is a community of humans, if we forced people to accept answers then people would have to accept answers that they don't like, or may be put off asking questions in the first place.
It's best just to think of the accept answer option as the question asker saying 'yes, solved the problem for me' and just rely on the voting score to know how useful other people found the answer. Often they're both the same anyway.