Starting from TailwindCSS v4, the npx tailwindcss command has been deprecated. The package has been split into several different parts, and the CLI, which isn't necessary for many users, has been moved to the @tailwindcss/cli package. It is now available as npx @tailwindcss/cli. See here:

• Get started with Tailwind CSS with Tailwind CLI - TailwindCSS v4 Docs
• TailwindCSS v4 Alpha: Using the CLI
• Problem installing TailwindCSS after "npx tailwindcss init -p" command - StackOverflow