Postcss and autoprefixer not working

[k1nxx]

Commands ran

npm init -y
npm i postcss postcss-cli -g
npm install postcss-cli autoprefixer
// npx postcss *.css --use autoprefixer -d build/ -- this returns an error
npm postcss *.css --use autoprefixer -d build/ 

npm postcss *.css --use autoprefixer -d build/ Returns

npm info it worked if it ends with ok
npm info using npm@6.14.12
npm info using node@v14.16.1

Usage: npm <command>

where <command> is one of:
    access, adduser, audit, bin, bugs, c, cache, ci, cit,
    clean-install, clean-install-test, completion, config,
    create, ddp, dedupe, deprecate, dist-tag, docs, doctor,
    edit, explore, fund, get, help, help-search, hook, i, init,
    install, install-ci-test, install-test, it, link, list, ln,
    login, logout, ls, org, outdated, owner, pack, ping, prefix,
    profile, prune, publish, rb, rebuild, repo, restart, root,
    run, run-script, s, se, search, set, shrinkwrap, star,
    stars, start, stop, t, team, test, token, tst, un,
    uninstall, unpublish, unstar, up, update, v, version, view,
    whoami

npm <command> -h  quick help on <command>
npm -l            display full usage info
npm help <term>   search for help on <term>
npm help npm      involved overview

Specify configs in the ini-formatted file:
    C:\Users\ryant\.npmrc
or on the command line via: npm <command> --key value
Config info can be viewed via: npm help config

npm@6.14.12 C:\Program Files\nodejs\node_modules\npm

Nothin happens.. Help!

[RyanZim]

No need to install things globally; npm install postcss postcss-cli autoprefixer, then npx postcss *.css --use autoprefixer -d build/ should work. If it doesn't, please post the error.