It is well known that people lighto shabbas candles before(many are careful to light 18-20 minutes before) shikya and no later. However, the Kitzur Shulchan Aruch 75:1 holds one has until 30 minutes before the emergence of 3 medium stars. So in a pressing situation why cant one hold like the Kitzur Shulchan Aruch? I have seen poskim who write that after shekiya it is forbidden (Mishna Brurah) and discuss if a non-Jew can light in their stead (Biur Halacha).
There are two shkias - the regular shkia and Rabbeinu Tam's shkia, which is 58.5 minutes after the regular shkia. Since we are dealing with a safek issur kares, you must be stringent and therefore you may not light candles even a millisecond after the regular shkia, and to do so is a safek issur kares.