v0.4.0 will drop support for Tailwind CSS v2.0 and is intended for use with Tailwind CSS v3.0+.
To upgrade, install @tailwindcss/forms@next:
npm install -D @tailwindcss/forms@next
This is an alpha release, so some things might be broken. I don't know about them otherwise I would have fixed them, but you are gonna find them! So try it, break it, and tell me about it when you do so we can get this thing really battle-tested for a proper v3.0 release in a few weeks.