Ok I solved it.
I know it's a old post, but I faced this problem lately and wandering for a solution I hit this unanswered post too, so I want to fill this.
If you don't see the "Merge" button in PullRequest is probably because, even as admin, you don't have permission to merge on that repo.
To solve this, as admin, go to repo Settings > Branch Permissions > [Click little pencil for edit Write Group] and add your user there. Once you have promoted you can merge then, and "Merge" button should appear accordingly.
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