Could not find asset in entry file error help

The following error:

Could not find app javascript: "/assets/foobar.js" in index.html. Found the following instead:
 - assets/vendor.js
 - assets/foobar.js

refers to Embroider not being able to find certain assets located in either your index.html or tests/index.html files. Generally, this occurs because you are missing {{rootURL}} from your index file such as:

Bad:

<script src="assets/vendor.js"></script>
<script src="assets/foobar.js"></script>

Good:

<script src="{{rootURL}}assets/vendor.js"></script>
<script src="{{rootURL}}assets/foobar.js"></script>

Embroider will be looking for the following asset so you will need to make sure that they exist in your index files:

<script src="{{rootURL}}assets/vendor.js"></script>
<script src="{{rootURL}}assets/APPNAME.js"></script>
<link integrity="" rel="stylesheet" href="{{rootURL}}assets/vendor.css">
<link integrity="" rel="stylesheet" href="{{rootURL}}assets/APPNAME.css">

Additionally, if you have configured outputPaths in ember-cli-build.js you may run into this error if that configuration does not point to a valid asset listed in the index file. However, outputPaths are not supported under Embroider so it is recommended that you migrate off of outputPaths and completely remove the configuration.