You seem to be using some v3 configs with some v4 configs at the same time.

Considering you want to use latest tailwindcss v4 with vite, you do not need to handle PostCSS manually.

Uninstall PostCSS, delete the PostCSS file and follow the steps here.

Your vite file seems to be correct.

Your index.css can lose the old v3 @tailwind directives in favour of the new @import "tailwindcss";