If you want to record MIDI from a session clip into a Digitakt track, make sure the Digitakt is set to receive MIDI notes (in MIDI settings), enable TRACK OUT for the Digitakt in Ableton MIDI settings.
Set the Ableton MiDI track output to Digitakt and the channel to correspond to the Digitakt Auto channel. Then select the track you want to record to on the Digitakt and live record from the clip (hit record and play).