I have just installed Emacs 27.9 on a new Mac running Monterey 12.0.1 (using https://emacsformacosx.com)

I then installed org-ref by putting the following lines in my .emacs:

(add-to-list 'load-path "~/.emacs.d/")
(load "org-ref-melpa.el")

For me the file "org-ref-melpa.el" is set up as follows:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; SETUP FOR ORG-REF ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

(setq user-emacs-directory "~/EMACS/org-ref/sandbox")

(require 'cl)
(require 'package)
(setq package-archives
      '(("org" . "http://orgmode.org/elpa/")
        ("gnu" . "http://elpa.gnu.org/packages/")
        ("melpa" . "http://melpa.org/packages/")))

(package-initialize)

(let ((packages (list 'org-plus-contrib 'org-ref)))
  ; refresh if needed.
  (unless (cl-every #'package-installed-p packages)
    (package-refresh-contents))

  (dolist (package packages)
    (unless (package-installed-p package)
      (package-install package))))

;; wrap lines
(global-visual-line-mode 1)

;; setup org-ref
(setq org-ref-bibliography-notes "~/EMACS/org-ref/notes.org"
      org-ref-default-bibliography '("~/EMACS/org-ref/references.bib")
      org-ref-pdf-directory "~/EMACS/org-ref/bibtex-pdfs/")

(unless (file-exists-p org-ref-pdf-directory)
  (make-directory org-ref-pdf-directory t))

;; Some org-mode customization
(setq org-src-fontify-natively t
      org-confirm-babel-evaluate nil
      org-src-preserve-indentation t)

(org-babel-do-load-languages
 'org-babel-load-languages '((python . t)))

(setq org-latex-pdf-process
      '("pdflatex -interaction nonstopmode -output-directory %o %f"
        "bibtex %b"
        "pdflatex -interaction nonstopmode -output-directory %o %f"
        "pdflatex -interaction nonstopmode -output-directory %o %f"))

(setq bibtex-autokey-year-length 4
      bibtex-autokey-name-year-separator "-"
      bibtex-autokey-year-title-separator "-"
      bibtex-autokey-titleword-separator "-"
      bibtex-autokey-titlewords 2
      bibtex-autokey-titlewords-stretch 1
      bibtex-autokey-titleword-length 5)

(require 'dash)
(setq org-latex-default-packages-alist
      (-remove-item
       '("" "hyperref" nil)
       org-latex-default-packages-alist))

;; Append new packages
(add-to-list 'org-latex-default-packages-alist '("" "natbib" "") t)
(add-to-list 'org-latex-default-packages-alist
             '("linktocpage,pdfstartview=FitH,colorlinks,
linkcolor=blue,anchorcolor=blue,
citecolor=blue,filecolor=blue,menucolor=blue,urlcolor=blue"
               "hyperref" nil)
             t)

;; some requires for basic org-ref usage
(require 'org-ref)
(require 'org-ref-pdf)
(require 'org-ref-url-utils)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

That is its the same as the version of the file found on:

https://kitchingroup.cheme.cmu.edu/blog/2015/12/22/org-ref-is-on-Melpa/

with minimal changes.

Now when I try to set up a bibliography file by typing for example:

bibliography:newref.bib

And then click on the link (as explained in the video in Kitchin's introductory video https://www.youtube.com/watch?v=2t925KRBbFc)

I get the error:

(wrong-type-argument stringp nil)

If I use the debugger: (setq debug-on-error t), I get:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  file-exists-p(nil)
  org-ref-get-bibfile-path(nil)
  org-ref-bibliography*-follow("newref.bib")
  org-link-open((link (:type "bibliography" :path "newref.bib" :format plain :raw-link "bibliography:newref.bib" :application nil :search-option nil :begin 88 :end 111 :contents-begin nil :contents-end nil :post-blank 0 :parent (paragraph (:begin 88 :end 114 :contents-begin 88 :contents-end 112 :post-blank 2 :post-affiliated 88 :parent nil)))) nil)
  org-open-at-point()
  org-open-at-mouse((mouse-2 (#<window 7 on manuscript.org> 97 (68 . 64) 140230978 nil 97 (9 . 4) nil (5 . 8) (7 . 14))))
  funcall-interactively(org-open-at-mouse (mouse-2 (#<window 7 on manuscript.org> 97 (68 . 64) 140230978 nil 97 (9 . 4) nil (5 . 8) (7 . 14))))
  call-interactively(org-open-at-mouse nil nil)
  command-execute(org-open-at-mouse)

I'm really very new to all things EMACS so if anyone out there can help me with this can you please give instructions that can be understood by a newb!

Thanks!!