I've got 62 Decillion on hand, and the game calls me a decillionaire, but the Steam Achievement hasn't unlocked. Am I doing something wrong?
Just continue playing and you will get it. Some people reported getting it at 10 decillion, others at 100 and some after next reset. No one reported that they were not able to get it at all.
So just keep playing until you get it.
I logged a ticket with the devs and they confirmed that this was a bug. Sadness!