So basically I developed a MMO without using any expansions and it sold over 200 million copies but I did not the achievement. Also the achievement does not show up in the in-game achievement list is that because it is hidden?
Also, will the achievement work if I continue on an old save , like was there a time this achievement wasn’t available and maybe that is why it didn’t work?
The Unobtanium achievement doesn’t include MMO’s (I think it’s said in the description). And yes, I believe it is hidden
@Haxor has it right!
This achievement pertains only to games, not MMOs, because the sales are based on subscriptions, not units sold. 
The achievement will indeed work at any point of a save, so if you want to try this again, here are a few pointers:
- A really high-seller will always be connected to industry/audience trends and the amount of hype that you’re getting
- Have a really good, balanced development process (make sure everyone gets enough rest!)
- Finally, it sometimes helps to pick up a little extra kick from making a sequel to a really successful/well rated game.
Hope this helps you out!