Incompatible units: 'rem' and 'px' - Bootstrap 4 and Laravel Mix

It happened to me too, it's like you said, the override of some variables is the cause of this error.

To fix this just comment this line in resources/assets/sass/_variables:

$font-size-base: 14px;

In the variables if you need are using other variables that laravel provides.

reference: https://github.com/twbs/bootstrap/issues/18368

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!