I have a Heroku deploy problem, currently, it’s failing on installing package.json and the error is

remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Phoenix app detected
remote:
remote: -----> Loading configuration and environment
remote:        Loading config...
remote:        WARNING: phoenix_static_buildpack.config wasn't found in the app
remote:        Using default config from Phoenix static buildpack
remote:        Detecting assets directory
remote:        WARNING: no package.json detected in root nor custom directory
remote:        * assuming phoenix 1.3.x and later, please check config file
remote:        Will use phoenix configuration:
remote:        * assets path assets
remote:        * mix tasks namespace phx
remote:        Will use the following versions:
remote:        * Node 6.9.2
remote:        Will export the following config vars:
remote: DATABASE_URL
remote: NODE_MODULES_CACHE
remote: POOL_SIZE
remote: SECRET_KEY_BASE
remote:        * MIX_ENV=prod
remote: -----> Installing binaries
remote: Resolving node version 6.9.2...
remote: Downloading and installing node 6.9.2...
remote:        Installing Node 6.9.2...
remote:        Using default npm version
remote:
remote: -----> Building dependencies
remote:        Installing and caching node modules
remote: npm WARN package.json @ No description
remote: npm WARN package.json @ No README data
remote:        npm ERR! addLocal Could not install /tmp/build_86e0734371e98eb49ec08fbf671e37ba/deps/phoenix
remote:        npm ERR! addLocal Could not install /tmp/build_86e0734371e98eb49ec08fbf671e37ba/deps/phoenix_html
remote:        npm ERR! Linux 4.4.0-1057-aws
remote:        npm ERR! argv "/tmp/build_86e0734371e98eb49ec08fbf671e37ba/.heroku/node/bin/node" "/tmp/build_86e0734371e98eb49ec08fbf671e37ba/.heroku/node/bin/npm" "install" "--quiet" "--unsafe-perm" "--userconfig" "/tmp/build_86e0734371e98eb49ec08fbf671e37ba/npmrc"
remote:        npm ERR! node v6.9.2
remote:        npm ERR! npm  v3.10.9
remote:        npm ERR! path /tmp/build_86e0734371e98eb49ec08fbf671e37ba/deps/phoenix
remote:        npm ERR! code ENOENT
remote:        npm ERR! errno -2
remote:        npm ERR! syscall open
remote:
remote:        npm ERR! enoent ENOENT: no such file or directory, open '/tmp/build_86e0734371e98eb49ec08fbf671e37ba/deps/phoenix'
remote:        npm ERR! enoent ENOENT: no such file or directory, open '/tmp/build_86e0734371e98eb49ec08fbf671e37ba/deps/phoenix'
remote:        npm ERR! enoent This is most likely not a problem with npm itself
remote:        npm ERR! enoent and is related to npm not being able to find a file.
remote:        npm ERR! enoent
remote:
remote:        npm ERR! Please include the following file with any support request:
remote:        npm ERR!     /tmp/build_86e0734371e98eb49ec08fbf671e37ba/assets/npm-debug.log
remote:  !     Push rejected, failed to compile Phoenix app.

I have tried https://help.heroku.com/18PI5RSY/how-do-i-clear-the-build-cache, also I have elixir-buildpack.config

elixir_version=1.8.2
erlang_version=21.2.5
assets_path=assets
phoenix_ex=phx

and phoenix_static_buildpack.config

clean_cache=true
compile="compile"
config_vars_to_export=(DATABASE_URL)
node_version=6.9.5
npm=3.10.8
phoenix_relative_path=.
remove_node=false

package.json is the default one

{
  "repository": {},
  "license": "MIT",
  "scripts": {
    "deploy": "webpack --mode production",
    "watch": "webpack --mode development --watch"
  },
  "dependencies": {
    "phoenix": "file:../deps/phoenix",
    "phoenix_html": "file:../deps/phoenix_html"
  },
  "devDependencies": {
    "@babel/core": "^7.0.0",
    "@babel/preset-env": "^7.0.0",
    "babel-loader": "^8.0.0",
    "copy-webpack-plugin": "^4.5.0",
    "css-loader": "^2.1.1",
    "mini-css-extract-plugin": "^0.4.0",
    "optimize-css-assets-webpack-plugin": "^4.0.0",
    "uglifyjs-webpack-plugin": "^1.2.4",
    "webpack": "4.4.0",
    "webpack-cli": "^2.0.10"
  }
}

but still no joy in solving the error above, did someone had something similar, how did you solved it? Thanks