Using vimscript plugin in lua neovim

I tried to use this simple script (its not from me) for simple reading a file and adding its content to a plugin called projectionist: https://github.com/tpope/vim-projectionist

(The original script: https://github.com/andyl/vim-projectionist-elixir/blob/master/ftdetect/elixir.vim)

I didn't find any api for reading a file in lua nvim. The error in the picture targets the line let l:json = readfile(s:proj_jsn) so I assume that this api is not available by lua?

if vim.g.loaded_vim_projectionist then
  return
end

vim.g.loaded_vim_projectionist = 1

vim.api.nvim_exec([[
  let s:base_dir = resolve(expand("<sfile>:p:h"))
  let s:proj_jsn = s:base_dir . "/projections.json"

  function! s:setProjections()
    let l:json = readfile(s:proj_jsn)
    let l:dict = projectionist#json_parse(l:json)
    call projectionist#append(getcwd(), l:dict)
  endfunction

  call s:setProjections()
]], false)

The error message (picture) I checked a several times if the file projections.json is in the right place, so this is not the mistake.

Thank you all in advance.