No worries at all. Did you by any chance use the download method ‘Grab binary packages directly’?
If yes, make sure to run the following before running any further commands in order to remove the broken installation:
apt-get -f install
In a debian stretch docker container I was able to install ‘owncloud-client’ version ‘2.2.4+dfsg-2’ with only the standard repositories enabled (/etc/apt/sources.list):
deb http://deb.debian.org/debian stretch main
deb http://security.debian.org/debian-security stretch/updates main
deb http://deb.debian.org/debian stretch-updates main
So you should be able to just run as root
apt-get install owncloud-client
If you need a more up to date desktop client (version 2.5.4.11654+oc-515) run the following commands (also as root):
wget -nv https://download.opensuse.org/repositories/isv:ownCloud:desktop/Debian_9.0/Release.key -O Release.key
apt-key add - < Release.key
echo 'deb http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Debian_9.0/ /' > /etc/apt/sources.list.d/isv:ownCloud:desktop.list
apt-get update
apt-get install owncloud-client
Hope that helps, let me know should you have any further questions.
Cheers,
Erik