Specifically, under "Note: To create a Poudriere jail for 12.0-CURRENT:", the command given assumes that head==12, which is no longer the case.
A commit references this bug: Author: carlavilla Date: Sat Nov 16 17:45:01 UTC 2019 New revision: 53598 URL: https://svnweb.freebsd.org/changeset/doc/53598 Log: Update the version of -CURRENT in the porters handbook PR: 240791 Submitted by: pauamma@gundo.com Patch by: carlavilla@ Approved by: bcr@ Differential Revision: https://reviews.freebsd.org/D22248 Changes: head/en_US.ISO8859-1/books/porters-handbook/testing/chapter.xml
Fixed.