"postcss" is not an internal or external command. postcss command not running. How to be??

Question

Windows

I'm learning BEM. Installed postcss, according to the tutorial on YouTube.

npm i  browser-sync --save-dev
                browser-sync start --server --files 'style.css'

I created the necessary configs and everything was fine, but when I got to
postcss style.post.css -o style.css
I got:

"postcss" не является внутренней или внешней
                   командой, исполняемой программой или пакетным файлом.

I do not know why.
The only moment when installing postcss and plugins (npm i postcss-cli autoprefixer postcss-nested --save-dev) I get the following:

npm WARN [email protected] No description
npm WARN [email protected] No repository field.
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: [email protected] (node_modules\fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for [email protected]: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})

It seems like a notification about optional things, but still.
Please help me to run the postcss command.

Answer 1

the postcss-cli package needs to be installed globally;
the -g flag just means global installation ;
when installed globally, the postcss command becomes available in the console