middleman: Slate - CssSyntaxError: /stylesheets/screen.css:5:1: Unknown word

Hello All,

I have just stared using slate and they use middleman and see you as contributors in their list. I hope this is the forum to ask my question. In case this is not the place, please let me know.

I’m trying to build the static content to host apidocs on a webserver. But I’m getting below error.

bundle exec middleman build --clean
== Sprockets will render css with SassC
CssSyntaxError: /stylesheets/screen.css:5:1: Unknown word
3 | 
@import
 'variables';
4 | 
@import
 'icon-font';

    5 | //
    @import
    'rtl'; // uncomment to switch to RTL format | ^ 6 | 7 | /*

bundler: failed to load command: middleman (/home/praveen/.rbenv/versions/2.6.5/bin/middleman)
ExecJS::RuntimeError: CssSyntaxError: /stylesheets/screen.css:5:1: Unknown word
3 | 
@import
 'variables';
4 | 
@import
 'icon-font';

    5 | //
    @import
    'rtl'; // uncomment to switch to RTL format | ^ 6 | 7 | /*

Input.error ((execjs):60175:16)
Parser.unknownWord ((execjs):62409:22)
Parser.other ((execjs):62014:12)

• Ruby version: ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-linux]
• Middleman version: gem middleman ~>4.3 gem middleman-syntax ~> 3.0 gem middleman-autoprefixer ~> 2.7 gem middleman-sprockets ~> 4.1
• OS version: Ubuntu Server 16.04

I hope I have provided enough information. Please let me know if you need any additional details.