Earlier I asked to have a question reopened in reopen question closed as duplicate. There was some debate and It was reopened. Thank you all who helped. So I decide to flag the reopen question finished, and was told that adding status-completed was a moderator only action. I believe this policy is slightly too strict. I believe that the original poster of unedited support requests like reopen questions should be able to mark them finished, after accepting a answer.
I think you're missing the point of that tag. As a question asker you can show your question has been resolved by either marking one of the answers as accepted or writing an answer up yourself and marking it. This is the mechanism for showing the issue's been resolved.
The status-completed tag is when new features are being worked on and/or implemented by moderators or StackExchange employees within the site or the software powering it. Using it in any other fashion would be confusing to the normal workflow of the Q&A format.