It does not matter wich version you choose to use. Either the version with a trailing slash or the version without the trailing slash should be indexable. So the version which should be indexed should also have a canonical tag reference coming from the other version.
Technically speaking version http://www.domain.de does not exist. The reason for this being each request must always be specified in a path. The smallest possible path is indicated between two „slashes“, „/„. Therefore, the smallest possible path of a domain is „http://www.domain.de/ (same goes for https).
But for Googles ranking it does not make a difference which version will be indexed,
Our crawler analyses the pages regarding this path with the slash, so we automatically add a trailing slash to the domain. Hence, your page with the trailing slash is referring with a canonical tag to the version with our, the tool correctly replies this page is not indexable. Regardless the version without is.