Welcome to the Wefugees Community and thank you for your question.
We discussed the so-called "Grenzübertrittsbescheinigung" already a couple of times on this platform. Please follow this link, for instance, for more details. You tagged "entry-ban" in your thread, but I assume you are not officially banned to enter the Schengen area for a certain period of time, am I correct? As you can also read in the recommended threads, the entry-ban is usually issued after deportations.
Unfortunately, I am not an expert on visa for the Voluntary Social Service ("Freiwilliges Soziales Jahr"/FSJ) for Germany. But I tried to make some research and found some useful information (including a requirements list) on the website of the German Embassy in Nepal, for example. On this website, however, you can find some contact details for counselling services and a search engine for respective vacant positions.
I hope it helps and please don't hesitate to get back to us with any further questions.
All the best,