Is there a possibility to run a functions on server. I am not talking about events but about functions
No but you can make a run-on-server event that calls the function 
I though about that as well. The problem is that I want my function to output something. Do you have any idea how to manage that?
That’s not how it works.
When you call a function to run on a different machine, the instruction is sent to the other machine over the wire. The other machine will only receive it several milliseconds later, meanwhile the calling machine still has to process and render game frames. These calls are asynchronous. When you call them from blueprint, they are put in a queue and return immediately (with obviously no return value), so your game code can continue executing.
If you want to “return” something from server, you can make a new run-on-client event, which you call from the server event once you are done there.
Thanks for the explanation. Helped a lot