Are achievements disabled? Press ESC, then click Achievements. It'll tell you why.

Achievements are disabled if:
- You have used a cheat
- You are playing Build mode
- You have changed the setup of the game (ie, you've pressed "show game options" when starting the game

Relatedly, could we get a "reset to default" option for the game options pane, which also re-enables achievements? I had to run through the tutorial a couple of times to get the achievement, because the first time I was curious about what game options there were.

I might have changed the game options the first time I played. Is there a way to reset it so I can earn achievments?

If you do that then you save is marked as "dirty" and you must start a new one.