It may have been a glitch. I can create new Aurora Postgres instances, with version 11.4 as the newest supported. RDS (non-Aurora) instances have support for 11.5.
None of the big 3 cloud vendors support PG 12 yet.
AWS generally is the first to provide support for newer versions, and they usually lag the PG release dates by 6-9 months for RDS and over 12 months for Aurora.