Output of running the command:

Short version:
Probably outdated postcss-next with old BrowserList version that doesn't support dead query, actually npm says postcss-next has been deprecated in favor of postcss-preset-env.
I don't want to eject to be able to use react-toolbox, and i guess i'm not the only one.
Probably related issues:
browserslist/browserslist#323
browserslist/browserslist#266
https://www.npmjs.com/package/postcss-cssnext