Error: PostCSS plugin autoprefixer requires PostCSS 8. Update PostCSS or downgrade this plugin

Quick fix

Downgrade your autoprefixer to version 9, use

"autoprefixer": "^9.0.0"

in your dev dependencies.

More details

PostCSS was updated to version 8, however, PostCSS CLI has not yet been updated to handle PostCSS plugins which use the new PostCSS 8+ API. Autoprefixer uses the new PostCSS 8 API since version 10.

This is documented under known issues in the PostCSS GitHub page.

Once PostCSS CLI is updated to handle plugins that use the new PostCSS 8+ API, this will likely not be an issue. But until then, you may need to downgrade some PostCSS plugins to avoid errors.

If you're having this problem and you're using Tailwind CSS v2, try this

npm uninstall tailwindcss postcss autoprefixer
npm install tailwindcss@npm:@tailwindcss/postcss7-compat postcss@^7 autoprefixer@^9

source: https://tailwindcss.com/docs/installation#post-css-7-compatibility-build

I am not sure about this but can you try installing postcss as a dependency?

npm i postcss

Ok, to me was fixed removing package-lock.json and installing:

"tailwindcss": "^1.8.10"
"postcss-cli": "^7.1.0"
"autoprefixer": "^9.7.5"

"dependencies": {
    "autoprefixer": "^9.8.6",
    "postcss": "^8.0.0",
    "postcss-cli": "^8.1.0",
    "tailwindcss": "npm:@tailwindcss/postcss7-compat@^2.0.2",
},

Dependecies object and version can be modified directly in the package.json file and it work