Issue not with Dietpi but with external source, however odd that may be.
Full instructions in github link above.
Try apt-get clean && apt-get update
If that doesn’t work you need to remove the following line from /etc/apt/sources.list
deb http://deb.debian.org/debian-security/ jessie/updates main contrib non-free
and then run apt-get update
DietPi-Software
─────────────────────────────────────────────────────
Mode: Installation completed
[ OK ] DietPi-Software | The system will now reboot.
This completes the DietPi-Software installation.