Hi. `ensure-package-description' procedure from emacs-build-system.scm contains an undefined elisp function `package--prepare-dependencies'. It was replaced with `lm--prepare-package-dependencies' after 29.1 release. You can see the changes here: https://cgit.git.savannah.gnu.org/cgit/emacs.git/commit/?id=bb5399e3cd75450db6db9b3c5829f7bd87ca1308