It appears that the repository has undergone several changes and renamings:

backend/node_modules/postcss/lib/warn-once.js

@@ -1,13 +0,0 @@
-/* eslint-disable no-console */
-'use strict'
-
-let printed = {}
-
-module.exports = function warnOnce(message) {
-  if (printed[message]) return
-  printed[message] = true
-
-  if (typeof console !== 'undefined' && console.warn) {
-    console.warn(message)
-  }
-}