For using SCSS in React you have to install node-sass firstly, so you can simply add it to your project by the command below:
npm install node-sass
# or
yarn install node-sass
Then you can freely change your CSS files to SCSS.
But what is the error says actually?
In your Layout.scss file you are using a variable which is $color-white and you didn't define it in the file properly, so you should either define in like this $color-white: #FFFFFF; in the same file (Layout.scss) or define it in the other file and import the file into your current file. For instance, you can create a file and name it as Constants.scss like this:
//Constants.scss
$color-white: #FFFFFF;
And then import it in the Layout.scss just like this:
//Layout.scss
import "./constants.scss"; // I Assume they are in the same directory.
Update
As if today the node-sass approach has been deprecated and it is recommended to use sass instead.
So it will be like this:
npm install sass
# or
yarn add sass