Cosmo Run

The description states this achievement unlocks after unlocking 20 other achievements in the game. Despite this, there are multiple reports of this achievement unlocking at various, seemingly random times during the game. Some report it unlocking with their last achievement (after 21 achievements), some while they only had 15-19 achievements unlocked, and some after 20 achievements while working on their last achievement. Some theorize this achievement unlocks after a certain time played, or after a certain cumulative score (such as 100,000, halfway to the 200,000 required), but the only consistent thing seems to be that it definitely doesn't unlock right when you unlock your 20th achievement.

The good news is that it does seem to unlock for everyone, just at different times. By the time you unlock all other achievements, you should have this one too. My personal experience was that this achievement unlocked at the same time as my last achievement, which was "Battlecruiser" and was my 21st achievement. So don't worry if/when it doesn't unlock with your 20th achievement.