I created a new simulation project called 'Mein erstes Projekt':
Kostenloses Projekt zum testen.
More of my public projects can be found here.
I see that your CAD is a shell and not a solid. The Tetrahedral mesh can only mesh solids.
See this forum post here: Can a tetrahedral mesh be created using geometry with only shells/faces?
Ali ( Tag me in the reply by using @Ali_Arafat )