Foundation gem Incompatible units: 'rem' and 'px'

i've installed the foundation gem with the version 5.5.2.1. when i start the application i recive this error:

Showing /home/user/Scrivania/sites/store/app/views/layouts/application.html.erb where line #9 raised:

Incompatible units: 'rem' and 'px'.

following some other case on the internet i tried to change the file _settings.scss on my gem folder:

../../var/lib/gems/2.1.0/gems/foundation-rails-5.5.2.1/vendor/assets/stylesheets/foundation/_settings.css

i've commented these lines:

$row-width: rem-calc(1000);
$column-gutter: rem-calc(30);
$total-columns: 12;

and i've set my custom $base-font-size:

 $base-font-size: 14px;
 $rem-base: $base-font-size;

i don't wont to downgrade my gems so can you help me with this error?

via Chebli Mohamed