This is an open issue over at the people of ngneat and needs to be resolved by them as well.

I will share the hotfix provided in the linked issue. All credits to kingjordan:

Thanks to @kingjordan for his solution.

Install the dependencies:

npm install tailwindcss@latest @ngneat/tailwind@latest

Create a configuration file for Tailwind CSS at the root of your workspace:

npx tailwindcss init

Add purge configurations:

module.exports = {
  ...
  purge: {
    enabled: guessProductionMode(),
    content: [ "<path/to/project/**/*.{html, ts, jsx, etc}>" ],
  },
  ...
}

Import Tailwind CSS styles into the root styles file of your project.

@import 'tailwindcss/base';
@import 'tailwindcss/components';
@import 'tailwindcss/utilities';

Just try to use the steps of tailwind install for angular version less then 11.2.0