Why that's the keystone? mission has gone

@aodoherty  The one time I had that mission, the same thing happened to me: my sims had traveled to France and retrieved the keystone, but the mission dropped when they arrived in China.  My sim found the mission on the adventure board right away though, and completed it while on that China trip.  I think it's just kind of borked in that it drops when your sims arrive in China.

Is there a reason your sims can't find the mission again and then start working on it right away?  That should be fine.  If you don't see it on the adventure board on day one, try doing a couple of easy missions first, or even assigning a mission and dropping it to refresh the options.  (This only works some of the time.)  Other than that, you can assign the mission with a mod whenever your sims are ready.

@puzzlezaddictThanks, so now I completed the Pangu's axe at the moment, then I have to go for hot springs, then I will try to get this adventure again using NRaas MC mod. It's not easy to remember all English title after changing the game language to other one. LOL

@aodoherty  Yup, it's MC that can grant an opportunity, and translating the names of things can definitely be annoying.  If you use NRaas Traveler, you can also pause the vacation temporarily to give your sims more time.  Click on the base camp tag or an in-game computer to see the option.

My case was bit different. After arrival to China, person to talk to was assigned. But opportunity anyway said - Go to China, and person had nothing to talk about except usual prases. Then I installed Master controler and Traveler mods. Dropped opportunity and got it through adventure board board (it was popping out time from time, but again, different persons were assigned, but non-talkative type), quickly went to France and back to China, with that opportunity active. And on my return from France, it worked.

Other variant (if this one would be unsuccessful), I was planning to drop and pick opportunity while in France (via board or via master controler), and go to China.

The only working solution I have found for this requires nraas debugenabler. If you have that installed you can go to China and when the adventure cancles (or if you started a new one go ahead and cancle it) click on your sim and select nraas - DE - Options: Smartphone (or Sim) - Opportunities - Add Opportunity - Adventures - China - Quests. This will bring up all the adventure opportunities for China. The one you will want is labled " EP1 Quest MaineChinaC1" and is the next step in the adventure after returning to China with the key

@saraandchester  MasterController works too; the command is under Basic > Opportunity: By Category > Adventure.  Sometimes one or two adventures don't show up, but selecting a new one (even if you don't accept it) should fix that.

Additionally, using DebugEnabler to assign a quest can wipe other quest data from the sim in question, or the entire household.  That would mean the active sims might keep receiving offers for opportunities they've already completed, including ones they might not be able to complete a second time.  So it's best to try other interventions first.

Having said all that, this thread is somewhat old, so I'll lock it here.