I had a terrible time figuring this out too, because the mobile app offers it and provides no feedback as to why it never succeeds. The reason is that Twitter only supports a very limited number of mobile carrier networks. Here's the support page listing them and you can see there are none for Germany: https://support.twitter.com/articles/20170024?lang=de#

All I can suggest is to keep trying every few months. I have been trying for years. My carrier still isn't listed but it finally started working about two weeks ago for me (I'm not in Germany though). Now I have to hope it keeps working with my still unlisted carrier or I risk getting locked out of my account. At least the bad guys are locked out too...