How to open PDF file with Org-ref?

Question

I have been trying to use the org-ref package with emacs and I copied the setup presented in the package where I placed the .bib files in a directory called ~/papers/ along with any pdfs of the papers in the same folder:

(use-package ivy-bibtex
    :straight t
    :init
    (setq bibtex-completion-bibliography '("~/papers/phd_ref.bib"
                                           "~/papers/masc_ref.bib")
          bibtex-completion-library-path '("~/papers/")
          bibtex-completion-notes-path "~/papers/"
          bibtex-completion-notes-template-multiple-files "* ${author-or-editor}, ${title}, ${journal}, (${year}) :${=type=}: \n\nSee [[cite:&${=key=}]]\n"

          bibtex-completion-additional-search-fields '(keywords)
          bibtex-completion-display-formats
          '((article       . "${=has-pdf=:1}${=has-note=:1} ${year:4} ${author:36} ${title:*} ${journal:40}")
            (inbook        . "${=has-pdf=:1}${=has-note=:1} ${year:4} ${author:36} ${title:*} Chapter ${chapter:32}")
            (incollection  . "${=has-pdf=:1}${=has-note=:1} ${year:4} ${author:36} ${title:*} ${booktitle:40}")
            (inproceedings . "${=has-pdf=:1}${=has-note=:1} ${year:4} ${author:36} ${title:*} ${booktitle:40}")
            (t             . "${=has-pdf=:1}${=has-note=:1} ${year:4} ${author:36} ${title:*}"))
          bibtex-completion-pdf-open-function
          (lambda (fpath)
            (call-process "open" nil 0 nil fpath))))

(use-package org-ref
    :straight t
    :init
    (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)
)
(define-key org-mode-map (kbd "C-c ]") org-ref-insert-cite-function)
(define-key org-mode-map (kbd "C-c r") org-ref-insert-ref-function)
(define-key org-mode-map (kbd "C-c l") org-ref-insert-label-function)

I am able to insert the citation using C-c ] and when I click on the citation, I can press p to open its pdf which I have also downloaded and kept in the same ~/papers/ folder. But when I press p , it does not open the pdf file and simply states Searching for program: No such file or directory, open. My goal was to click on the citation link and have org-noter open it up so I can start taking notes on the paper. So how can I configure org-ref to properly open up the pdf file?

Here is the error I receive:

Debugger entered--Lisp error: (file-missing "Searching for program" "No such file or directory" "open")
  call-process("open" nil 0 nil "/home/user/papers/huang91_optic_coher_tomog.pdf")
  (lambda (fpath) (call-process "open" nil 0 nil fpath))("/home/user/papers/huang91_optic_coher_tomog.pdf")
  org-ref-open-pdf-at-point()
  funcall-interactively(org-ref-open-pdf-at-point)
  call-interactively(org-ref-open-pdf-at-point)
  hydra--call-interactively-remap-maybe(org-ref-open-pdf-at-point)
  org-ref-citation-hydra/org-ref-open-pdf-at-point-and-exit()
  funcall-interactively(org-ref-citation-hydra/org-ref-open-pdf-at-point-and-exit)
  call-interactively(org-ref-citation-hydra/org-ref-open-pdf-at-point-and-exit nil nil)
  command-execute(org-ref-citation-hydra/org-ref-open-pdf-at-point-and-exit)