ownCloud was removed from Debian

ownCloud was removed from Debian. If you want to use it you have to migrate to the official ownCloud repository: post

--- From apt show owncloud-client:

Description: folder synchronization with an ownCloud server - GUI

Installing Owncloud server in Debian 9

wget -nv https://download.owncloud.org/download/repositories/10.0/Debian_9.0/Release.key -O Release.key
sudo apt-key add - < Release.key
sudo nano /etc/apt/sources.list.d/owncloud.list

add this line

deb http://download.owncloud.org/download/repositories/stable/Debian_9.0/ / 

and then:

sudo apt-get update
sudo apt-get install owncloud-files