If you have skiped directly from the end of KOTET to Ossus
whithout doing the traitor arc you'll not be able to propose

So after KOTET, you need to play "Fractured Alliance"
which is Iokath start this from you ship's terminal,
then play "solo" through the Umbara, Copero and then Nathema FPs that advance the story,
and at the end of the traitor arc, you should have the option to ask Lana to marry your character.

Here's a vid of the proposal