Hi. I was just able to fix this. Heroku doesn't seem to mix well with Webpack as it tries to find
bootstrap/scss/bootstrap in the
app/assets/stylesheets folder. You have to point the import out of the folder to node_modules where Bootstrap stored in.
Make sure that your
app/assets/stylesheets/application.css is renamed to
application.css.scss and change
@import "bootstrap/scss/bootstrap"; to