The template already has a prefix "Vorlage"
Since making it, you set the area to only allow pages with no prefix.
When trying to save it without a prefix, VaultWiki detects there is already a wiki page called just "Gugg-Buk"
VaultWiki will not let you make two pages with the same name, so you must pick a different name.
This is expected behavior. If you do not want to rename your page, then you will have to allow the "Vorlage" prefix in this area.