Node app in Heroku: Module build failed: Error: Unexpected "space" found

I have a node app that has been live on Heroku for a year, but the last update is now crashing on build. I haven't seen this error before and can't find any help.

Here is the error log:

       yarn run v1.12.3
       $ react-scripts build
       Creating an optimized production build...
       Failed to compile.

       ./src/components/Admin_Navigation/navigation.css
       Module build failed: Error: Unexpected "space" found.


error Command failed with exit code 1.
       info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! [email protected] heroku-postbuild: `yarn build`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the [email protected] heroku-postbuild script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
npm ERR! A complete log of this run can be found in:
npm ERR!     /tmp/npmcache.22xgq/_logs/2018-11-22T00_36_38_522Z-debug.log
-----> Build failed

       We're sorry this build is failing! You can troubleshoot common issues here:
       https://devcenter.heroku.com/articles/troubleshooting-node-deploys

       Some possible problems:

       - Dangerous semver range (>) in engines.node
         https://devcenter.heroku.com/articles/nodejs-support#specifying-a-node-js-version

       Love,
       Heroku

 !     Push rejected, failed to compile Node.js app.
 !     Push failed

I've tried the following:

1. Update Yarn Lock
2. Update package.JSON
3. Updated Yarn & Node versions

Any of your thoughts are appreciated. Thanks

Update: this is caused by an unnecessary space in CSS. You can manually look through the CSS for the space, or a less optimal solution is to roll back Node to a version that is less syntax-rigid.