The Romulan mission, Promotion: Sub-Commander, indicates to talk with Admiral Kererek on New Romulus Command Center. However, when you go to the Command Center the mission text details vanish and talking with Admiral Kererek results in no new conversations or missions completion.
This is a long standing bug that has not been addressed as per:
https://steamcommunity.com/app/9900/discussions/0/1621726179570808643/ (2018)
https://www.reddit.com/r/sto/comments/7z8avb/bugcant_complete_the_promotion_subcommander/ (2018)
https://www.reddit.com/r/sto/comments/5n8mb4/cant_complete_promotion_subcommander_admiral/ (2017)
I have completed all suggestions in the forum and none of them appear to resolve the issue.
Comments
https://www.arcgames.com/en/forums/startrekonline#/discussion/1218324/romulan-promotion
Wrong promotion in question. The one indicated in that posting is for Centurion and not Sub-Commander. The way to fix Centurion does not work for Sub-Commander promotion bug.
My character is level 22 now and is on level 22 missions which is above the requirement for Sub-Commander of level 20.