Have you created the project already?

In order to attach a Module to a project, you need to create it first:

npx create-next-app@latest my-app
cd my-app

The package.json is going to be created, along with all other necessary files. Then you can run the commands to install the modules you want.

If the project already exists, make sure to run the command on the root directory (Where the package.json is located). Ex:

cd C:Documents\Next\my-app
npm install tailwindcss @tailwindcss/cli