well you can do this as root as well 
I guess if you enable the user, it should be available again. BTW you could just run ncc command. There you will see all available options. As this is all NextCloud specific, it might be good to check as well on NextCloud websites. These guys are more specialist on NextCloud questions as we are on this Board The only difference is that on DietPi you can use ncc command instead of occ.
For reference https://dietpi.com/forum/t/dietpi-software-details-for-all-installation-options/22/75
You may want to configure your Nextcloud from command line via occ command: > Nextcloud admin manual
We added a shortcut to the otherwise necessary '> sudo -u www-data php /var/www/nextcloud/occ> ':
Just use '> ncc> ’ followed by the desired command inside your terminal.