Build Failure on Heroku

We have been running an Elixir based site on Heroku for quite some time. We make updates to the site every few months. We last made an update back in December 2018 and did not have any issues. Unfortunately, we recently tried to publish a new update for our site and ran into a strange build error. We've tried a number of things to resolve. We did not change anything with respect to the Heroku stack or build packs. Here is the error:

npm ERR! Linux 4.4.0-1038-aws
npm ERR! argv "/tmp/build_938959a090592f7c5e7bf4b1770b7816/.heroku/node/bin/node" "/tmp/build_938959a090592f7c5e7bf4b1770b7816/.heroku/node/bin/npm" "run" "deploy"
npm ERR! node v6.9.2
npm ERR! npm v3.10.9

   npm ERR! missing script: deploy
   npm ERR!
   npm ERR! If you need help, you may report this error at:
   npm ERR!     <https://github.com/npm/npm/issues>
   
   npm ERR! Please include the following file with any support request:
   npm ERR!     /tmp/build_938959a090592f7c5e7bf4b1770b7816/npm-debug.log

! Push rejected, failed to compile Phoenix app.
! Push failed

This is on Heroku's cedar-14 stack. Grateful for any help!

The issue names no repository files, tests, or entry points; begin with the Heroku build output, especially the npm run deploy and missing script error. Done means identifying a reproducible cause and documenting or validating a fix that lets the Phoenix app build successfully on Cedar-14.