Since I couldn't find any solution to this (just found another person with the same bug, but no answer) I'm posting this thread.

This quest is that one that starts on China, then you have to go to France to retrieve a keystone on Landgraab Chateau and then go back to China to proceed with the quest.

My sim could retrieve the keystone and everything was fine, she came back home, waited for 2 sim-days and then traveled to China to proceed with this quest. Arriving there, a message pops up saying "you cannot finish the chain because the target sim is unavailable".
I thought that the target sim died, but no, he was alive.
I tried to cancel the opportunity and taking it again, and it said to go to China. I already was on China so I came back home and traveled to China again. Then the message saying that the targed sim was unavailable popped again.
I tried this over three times and the same happened, the game just points me to another sim and says that they are unavailable. And they're not stuck on anywhere, I went to the house of one of thoses sims and got to talk to him and stuff,

I don't believe that the CC/mods I have are causing this, I have few CC installed (stuff that I had on my old laptop and never gave me any trouble) and few mods like Nraas Master Controller and Overwatch, and few tuning mods that aren't related with WA.

There are several reasons I can think of that may cause this:

(1) Your own Sim was hard-reset (manually or through ErrorTrap) some time before you went back to China?
(2) IIRC this quest is given by the special merchant. Does the Sim you're supposed to talk to still have that role?
(3) Those cross-world quests are always a bit glitchy. Do you still have the save from before you went back to China first? I always save before I travel and keep that save because of this. Sometimes reloading that save and travel again, and again, and again can suddenly result in the quest triggering correctly. I recently had this with the Nectaux tomb quest that requires that you go to Egypt from France. I had to try three times until my arrival in Egypt was successful.

Well, I had reset the town (Sunset Valley) with MC when my sim came back from China, before going to France.
But if it was this, wouldn't the quest get this glitch on France?

Hard to say. I just know that a hard reset can damage open opportunities and LTW counters, because a hard reset destroys the Sim and replaces it with an exact copy. That's why it is a good idea to always put your current household and lot on the exception list of MC's Reset Everything. Personally I have stopped resetting towns and even hard reset Sims only as a last resort.

This:
suggests to me that the game is confused on who your Sim and its whereabouts are, something I've seen as one consequence of a Sim having been reset while an opportunity was still open.

But, yes, I'd think, too, that the quest should then have already been broken in France, but this is The Sims. Shit happens and it happens around every possible corner.

So, the quest worked while in France (step 1), you got the key and the opportunity tab then said to go back to China (step 2). You arrived in your homeworld (step 3) and the opportunity tab still said said you should travel to China (step 4). On arrival in China (step 5) the quest gets dropped. If you haven't got the save from step 4 (in homeworld with the order to go to China) on your hard drive anymore, hm, what you could try is to go back home, cancel the opportunity there, then travel to China with the key in your inventory and then use MasterController to give your Sim the exact opportunity within the quest chain they should get when they arrive in China with the key. Usually the titles of these quests should help you in locating it in MC. So, basically do not start the whole quest chain again but only the step where the game failed previously but have the Sim travel to China without any open opportunity first.

I had a save right after my sim came back from China and that didn't get the hard reset (I know that since on that hard reset the stuff stored on one of those alchemy shelves just gone).
I traveled to France again, grabbed the key again and saved there, before going home.
Then came back home, traveled back to China, and the bug happened again

I will try what you suggested, Don.

As I did that quest chain a few weeks ago I can at least say that this is not a universal bug. While there are quests that are terminally and reproducibly broken this one isn't.
But just now I remember that another thing in that quest was broken, however. The tomb itself that you will have to enter once you get the key part in China working will perhaps have unroutable areas in the crucial spots now, meaning that you cannot complete the final stages of the quest without altering the tomb in BuildMode.

I have bad news.
I did as you said, cancelled the opportunity before going to China, and when I arrived there I opened MC to give the opportunity, but the only one related was "Knowledge of the Past" itself. And selecting this made the whole quest reset to the start, even the key from France just gonw from my sim's inventory.

Maybe I'll try to finish this quest with another sim to see if the bug persists.

I always thought that the point of making you go to China was to make sure that you had an opportunity to learn Martial Arts so you can Zeneport around the obstacles. Or just use Debug "Teleport here"

The, new, problems in that tomb cannot be solved with teleporting, unfortunately. You have to interact with objects like floor switches, you cannot teleport onto a floor switch and even if you manage the Sim refuses to activate the switch with the route fail bubble. I had to enable testingcheats and whenever I encountered an unreachable trigger went into shift/click and "Build/Buy on this Lot" and either knock out a wall, move an obstacle or move the trigger, hoping that the links would not break.

I found routing issues on Temple of Heaven too, on the path to get the tomb symbol there was two diagonal walls and my sim couldn't walk there, I rebuilt the wall with testingcheatsenabled true and only got to walk there by teleporting.

Yeah, that has been there since Late Night, I think, and it got even worse later.