grunt build error

[bowenjnr]

When I try to serve or build the frontend I get the error below;

Running "postcss:server" (postcss) task
Unknown error from PostCSS plugin. Your current PostCSS version is 6.0.23, but autoprefixer uses 4.1.16. Perhaps this is the source of the error below.
Unknown error from PostCSS plugin. Your current PostCSS version is 6.0.23, but autoprefixer uses 4.1.16. Perhaps this is the source of the error below.
Fatal error: css.eachAtRule is not a function