As of v16.1 TurnKey Nextcloud includes a 'turnkey-occ' command (a simple wrapper around 'occ'). If you aren't on v16.1, then you can download it (and make it executable) like this:

URL=https://raw.githubusercontent.com/turnkeylinux-apps/nextcloud/master/overlay
FILE=usr/local/bin/turnkey-occ
wget -O /$FILE $URL/$FILE
chmod +x /$FILE

Then you can use it instead of 'occ'. I.e to run the command you noted, this should work:

turnkey-occ config:app:set files max_chunk_size --value 0

Alternatively, if you'd rather use occ directly, you can use 'su', like this:

cd /var/www/nextcloud
su - www-data -s /bin/bash -c "php occ config:app:set files max_chunk_size --value 0"

Or finally, if you'd rather use 'sudo', install it first:

apt update
apt install -y sudo

Then you should be able to use it as per your original post.

If you continue to have issues, please post the appliance version you are using (if unsure, post the output of 'turnkey-version'), where it's running and any error messages you are seeing.