This is resolved for DietPi v107: https://github.com/Fourdee/DietPi/issues/175
The package in the Raspbian repo has changed and requires IPv6 to install.
You either need to enable IPv6 system wide, or, disable IPv6 in /etc/nginx/sites-available/default
#listen [::]:80 default_server;
Then, reconfigure:
dpkg --configure nginx