We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser.
As you can see the whole Planets & Sectors tab is blank, and on the planet screen there's no box to click to assign a governor for my (apparently non-existent?) sector.
Is it possible yopu are loading a save from an older version into the current version? If so that would explain it, you need to revert to the older game version in the Steam Betas tab for this game.