Hi, shawn! By default, Heroku has the production flag set to true on NodeJS projects, which results in the installation of the production dependencies only. If you would also like to install devDependencies, you can either set the corresponding production flag to false...
$ heroku config:set NPM_CONFIG_PRODUCTION=false
...or optionally, move all of your npm devDependencies to dependencies; as is recommended in the story which Sandeep shared in his answer. Please do give it a complete read for a quick checklist of things to do, to get the MERN boilerplate deployed to Heroku.