Node Sass version 5.0.0 is incompatible with ^4.0.0.

C++
with npm =>
1.npm uninstall node-sass
2.npm install [email protected]
with yarn => 
1.yarn remove node-sass
2.yarn remove [email protected] npm =>
1.npm uninstall node-sass
2.npm install [email protected]
with yarn => 
1.yarn remove node-sass
2.yarn add [email protected] npm =>
1. npm uninstall node-sass
2. npm install [email protected]
with yarn => 
1. yarn remove node-sass
2. yarn add [email protected]

// ^ Update to MR J's post    npm uninstall node-sass
    npm install [email protected]

Or, if using yarn (default in newer CRA versions)

    yarn remove node-sass
    yarn add [email protected]
1 Uninstall node-sass
npm uninstall node-sass

2  use sass by:
npm install -g sass
npm install --save-dev sass
npm uninstall node-sass
npm install [email protected]
Source