Let's Learn Big Numbers achievement not unlocking
Solution 1:
For me it triggered somewhere random, but most people get it around 10 decillion, something I can find in more places on the internet, for example here or here:
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.
Solution 2:
I logged a ticket with the devs and they confirmed that this was a bug. Sadness!