So I've won the war, but when i force the end of it I can't select any of the territories I've occupied? I hadn't played for a few months but before we used to be able to choose that, even vassalize the enemy, and now i returned to the game with any of these options. Am I missing something?

It doesn't look like you're forcing a surrender on Semiramis, but that she is willingly surrendering to you in an attempt to end the war early (I'm surprised she can do this at 0 war support!).

Either side can choose to surrender (with the awkwardly phrased "Offer Surrender" button), attempting to end the war on their own terms.  In the screen above, you should be able to Refuse her surrender and then force a surrender on your own terms, though I'd save the game first to be sure I'm understanding this properly.

You are absolutely right! For some reason it came straight with the AI's offer, I had to refuse it and then i could click on force surrender again and set my terms! Thanks!

Glad to hear it worked!  Good luck in the rest of the game. :D

Glad as well that works hahah

But yep the AI send you "their" terms to surrender to you (of course with good terms for them), but you can impose yours ^^

If I remember properly (from my own attempts to offer surrender), the surrenderer must give in to the victor's demands as part of their "surrender package".  I'm a bit surprised to see none in Semirarmis' offer.  Obdestroyer, did you not have any demands when the war started?   (I suppose it's more common than I think; in my own play style, I try not to build up demands unless I'm pursuing war.  That means an enemy who declares on me means that I have no demands against them)