Symbol’s value as variable is void: tag-name

Troubleshooting
14

I had exactly the same issue, thanks for reminding me about ;; -- lexical-binding: t; --.

I use org file as my init file, so I had to put THIS at the very top of my init file:

#+BEGIN_SRC emacs-lisp
;; -- lexical-binding: t; --
#+END_SRC

And now everything in this video works as expected.