The occ console is located in the Nextcloud sub-directory, so you need to change to that directory first (so customize the path in the following cd command to lead to the NextCloud sub-directory):

cd /var/www/PATH_TO/nextcloud

If you don‘t know the exact name of the app that you plan to disable, you can list all apps with:

sudo -u www-data php occ app:list

Remember, the occ console is located in the Nextcloud directory, so make sure that you change to that directory first before issuing the command to disable the app (customize the path to lead to the nextcloud sub-directory):

cd /var/www/PATH_TO/nextcloud

sudo -u www-data php occ app:disable NAME_OF_APP