Incompatible units: ‘rem’ and ‘px’ – Bootstrap 4 and Laravel Mix

Solved

1. remove the bootstrap entry from package.json and replace it with
   “bootstrap”: “4.0.0-alpha.6”, in resources/assets/sass/app.scss,
2. comment out the import of variables. change the path of bootstrap to
   @import “node_modules/bootstrap/scss/bootstrap.scss”;
3. in resources/assets/js/bootstrap.js, look for require(‘bootsrap-sass’);
   and change it to require(‘bootstrap’);

Link!