Remember when you first setup elpaca in your init.el and you copy+pasted a big block of elisp from elpaca’s documentation? That elpaca setup code has a version-number variable that it sets. When you initialized your new Emacs instance, it installed elpaca from MELPA, but the version on MELPA is a newer version than your copy+pasted code from elpaca’s docs. The solution is to replace your copy+pasted code by copy+pasting the new code in elpaca’s docs.
Ah, right. That’s obvious in hindsight, but I didn’t give it much thought.