As already mentioned you need to run `npx create-sitecore-jss nextjs`.

This is a new feature that was added to `Sitecore JSS 20.0.0:` https://github.com/Sitecore/jss/releases/tag/v20.0.0

> New Features & Improvements [create-sitecore-jss]
> 
> - New package containing initializers / templates for JSS sample applications (#881), (#883), (#882), (#880), (#879), (#878), (#876)
> - This replaces previous `jss create` method to create new JSS applications
> - You can now create an application with guided prompts using initializer command of choice (`npx create-sitecore-jss`, `npm init`
> `sitecore-jss`, `yarn create sitecore-jss`) 🎉🎉🎉

So that is why `jss create test-app nextjs` works when trying to use `Sitecore JSS 19.0.0`. It doesn't work anymore with `Sitecore JSS 20.0.0`.