There is a scenario that you should aware of it :
1) in a post, if I mark it as solved, the button "set as answer" exists until next refresh.
2) so if I click on this button, Site returns error : "You don't have this permission"
I think this error confuses the Site Admin because it's irrelevant. maybe somethings like this could be better :
Your post is already solved and you can not select an answer as correct one. If you want to do this mark the post as unsolved and then select correct answer!