Error: Undefined variable: "$AU-color-background-alt-shade-suggestion".

I'm getting the following error when trying to compile the theme's assets.

[02:53:46] Starting 'default'...
[02:53:46] Starting 'serve'...
[02:53:46] Starting 'css'...
[02:53:46] Starting 'scripts_uikit'...
[02:53:46] Starting 'images'...
[02:53:46] Starting 'removeSourceMaps'...
[02:53:46] Starting 'scripts'...
[02:53:47] gulp-imagemin: Minified 0 images
[02:53:47] Finished 'images' after 1.02 s
[02:53:47] Finished 'removeSourceMaps' after 1.02 s
[02:53:47] Finished 'scripts' after 1.48 s
[02:53:47] Starting 'removeTemporaryStorage'...
[02:53:47] Finished 'scripts_uikit' after 1.51 s
[02:53:47] Finished 'removeTemporaryStorage' after 14 ms
[02:53:48] gulp-notify: [Gulp] Error: assets/scss/components/navigation/_menu__main.scss
Error: Undefined variable: "$AU-color-background-alt-shade-suggestion".
        on line 6 of assets/scss/components/navigation/_menu__main.scss
        from line 33 of assets/scss/style.scss
>> w: inset 0 $main-menu-border-negative 0 0 $AU-color-background-alt-shade-sug
   ------------------------------------------^

[02:53:48] gulp-notify: [Error running notifier] Could not send message: not found: notify-send
[02:53:48] Finished 'css' after 2.83 s
[02:53:48] Finished 'serve' after 2.83 s
[02:53:48] Finished 'default' after 2.84 s

I downloaded the theme into my project and followed the instructions in the readme to set up the theme.

Not sure if this is related but I'm running the commands within a docker container.

govCMS / govcms8_uikit_starter

Error: Undefined variable: "$AU-color-background-alt-shade-suggestion". #66