Prisma can't connect with an IPv6 host

If you haven’t deployed your app yet then you can do so with fly launch I believe. Starter guides are here: Language & Framework Guides · Fly Docs

Otherwise, if you can’t do fly ssh console because your app instance is failing, there’s not much I can really suggest other than checking logs + related posts.

Also for the 4th step, do I understand it right that Fly will handle the “replacing”? Can we also use the internal.postgres_app_name address all along or are there some limits?

Fly won’t handle the replacing, you’ll need to manually copy the DATABASE_URL and manually replace the IPv6 address with YOUR_POSTGRES_APP_NAME.internal