There is already an answer on this:
https://mail.ibr.cs.tu-bs.de/pipermail/ibr-dtn/2014-February/000623.html

In detail, your Pi does not support IPv6, but the daemon tries to
utilize it. Calm down and check if something you expect is not working.
If yes, ask again with more details. If not ignore the message.

If dtnping fails, first check if the clocks are in sync. For dtnping
both clocks must not be more than 20 seconds apart.