How to Fix "npm ERR! addLocal Could not install" - npm

Question

I'm getting this error when attempting to push my Phoenix application to Heroku.
I've been following the guide from the Phoenix framework website.
What is causing this error and how can I fix it?
WARNING: phoenix_static_buildpack.config wasn't found in the app
remote: Using default config from Phoenix static buildpack
remote: Will use the following versions:
remote: * Node 6.9.2
remote: Will export the following config vars:
remote: * Config vars DATABASE_URL
remote: * MIX_ENV=prod
remote:
remote: -----> Installing binaries
remote: Downloading 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 ERR! addLocal Could not install /tmp/build_23f60aac277eab15b4ce274dcaf9bb16/deps/phoenix
remote: npm ERR! addLocal Could not install /tmp/build_23f60aac277eab15b4ce274dcaf9bb16/deps/phoenix_html
remote: npm ERR! Linux 3.13.0-105-generic
remote: npm ERR! argv "/tmp/build_23f60aac277eab15b4ce274dcaf9bb16/.heroku/node/bin/node" "/tmp/build_23f60aac277eab15b4ce274dcaf9bb16/.heroku/node/bin/npm" "install" "--quiet" "--unsafe-perm" "--userconfig" "/tmp/build_23f60aac277eab15b4ce274dcaf9bb16/npmrc"
remote: npm ERR! node v6.9.2
remote: npm ERR! npm v3.10.9
remote: npm ERR! path /tmp/build_23f60aac277eab15b4ce274dcaf9bb16/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_23f60aac277eab15b4ce274dcaf9bb16/deps/phoenix'
remote: npm ERR! enoent ENOENT: no such file or directory, open '/tmp/build_23f60aac277eab15b4ce274dcaf9bb16/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_23f60aac277eab15b4ce274dcaf9bb16/npm-debug.log
remote: ! Push rejected, failed to compile Phoenix app.

Answer 1

I was having the same issue as you before. Try adding the npm version to the phoenix_static_buildpack.config file
Here's a sample from my app:
phoenix_static_buildpack.config (in the root)
clean_cache=false
compile="compile"
config_vars_to_export=(DATABASE_URL)
node_version=6.9.5
npm=3.10.8
phoenix_relative_path=.
remove_node=false
You can also set the
elixir_buildpack.config (also in the root directory)
erlang_version=18.3
elixir_version=1.4.0
always_rebuild=false
runtime_path=/app