You can lookup the (wrong) user/group by executing:
cd /var/www/html/nextcloud/apps
ls -l
If you just want to fix it, execute:
chown -R www-data:www-data /var/www/html/nextcloud/apps
If necessary replace www-data
by the webserver user on your system (“www-data”, “http”, “apache”, …), although www-data should be the right choice on Raspbian.