Disclaimer: You may not have the same issue if you are reading this after a while from 01/10/2021
I was doing a handful of Ghost installations today (01/10/2021) and noticed that all the installations stuck at obtaining an SSL certificate from Let’s Encrypt.
I have no idea why this is happening but most probably this may have something to do with Let’s Encrypt CA changes due in late September.
If you have the same issue (and need a workaround), follow the below link and instructions.
Note: that if you are installing Ghost on a different configuration than Ubuntu 20.04 and Nginx, you need to change those options in the instructions before proceeding further.