Auctex requires a :pre-build step which involves generating elisp files. If that step fails, certain parts will break. There are several ways to configure the build as well. See the various issues on the bug tracker for more info. e.g,

https://github.com/progfolio/elpaca/issues/191

This is how I got it to work

```
(use-package tex
  :demand t                             
  :elpaca (auctex :pre-build (("./autogen.sh")
                              ("./configure"
                               "--with-texmf-dir=$(dirname $(kpsexpand '$TEXMFHOME'))")
                              ("make")))

... rest of your config)
```

Here is the solution I came up with, following the hints given by [[https://github.com/progfolio/elpaca/issues/191]]

 (defun ded:elpaca-build-dir (p)
    "Return the elpaca build directory for package symbol p"
    (-first-item
     (f-directories elpaca-builds-directory
  	                (lambda (dir) (string-match-p (concat "^" (symbol-name p) "$") (f-filename dir))))))

  (use-package auctex
    :elpaca (auctex :pre-build (("./autogen.sh")
                                ("./configure" "--without-texmf-dir" "--with-lispdir=.")
                                ("make")
                                ("install-info" "doc/auctex.info" "doc/dir")
                                ("install-info" "doc/preview-latex.info" "doc/dir")))
    :mode (("\\.tex\\'" . TeX-latex-mode)
           ("\\.tex\\.erb\\'" . TeX-latex-mode)
           ("\\.etx\\'" . TeX-latex-mode))
    :init
    (add-to-list 'Info-additional-directory-list (f-join (ded:elpaca-build-dir 'auctex) "doc"))
    (add-hook 'tex-mode-hook
              (lambda ()
                (load "auctex.el")
                (setq TeX-command-extra-options "-shell-escape")))
    :config
    (setq-default TeX-global-PDF-mode 1)
    (setq-default  preview-scale-function 1.5)
    (setq TeX-auto-save t
          TeX-parse-self t
          default-truncate-lines t
          TeX-save-query nil
          TeX-source-correlate-method 'synctex))