If you have any mods enabled, it will disable your ability to get achievements. If you have a city that you started using mods on, then that city is "dead". Your city must never have the taste of mods if you want achievements. I suggest having a separate city for achievements and naming it something so that you know to disable mods when playing on it.
All you have to do for this achievements is build a Sauna.
1) Click the Healthcare button on the bottom (Red medical looking one).
2) Choose the Sauna.
3) Build the Sauna somewhere.
That's all there is to it!
Some things to keep in mind:
- The Sauna is unlocked with you get to the level of "Big Town". AKA, your city has a population of 7,500.
- The Sauna is 400 a week for upkeep.
- The Sauna is 25,000 to purchase.