I've had this error for over 24 hours now. Unlike this question, I only have the 4 A records and 1 CNAME record, which point to github's 4 addresses and myusername.github.io. There's no parking A record which could be causing the error. I've followed all the instructions outlined from github and namecheap
What's preventing github from provisioning my TLS certificate?
Turns out it was a glitch from Github. After switching the domain from www.domain.com to domain.com then back to www.domain.com twice, the certificate was provisioned.