Board Thread:General Discussion/@comment-173.31.160.159-20181004014619/@comment-26213507-20181004020958

You have to do five radiant quests in each of the main cities/holds (Whiterun, Solitude/Haafingar, Windhelm/Eastmarch, and Markarth/The Reach). After that, (if Mercer Frey is dead, and the questline complete) the option should appear when talking to Brynjolf, and the promotion should commence.