The client 2.4 will not install on a debian 9.3 64-bit. Here is the results of the apt-get update:
Hit:1 http://security.debian.org/debian-security stretch/updates InRelease
Hit:2 https://deb.opera.com/opera-stable stable InRelease
Ign:3 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Debian_9.0 InRelease
Hit:4 http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Debian_9.0 Release
Ign:6 http://ftp.us.debian.org/debian stretch InRelease
Hit:7 http://ftp.us.debian.org/debian stretch-updates InRelease
Hit:8 http://ftp.us.debian.org/debian stretch Release
Reading package lists...
So far everything is looking good. However, after running "apt-get install owncloud-client" I get the following:
Reading package lists...
Building dependency tree...
Reading state information...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
owncloud-client : Depends: libowncloudsync0 (= 2.4.0-8911) but it is not going to be installed
Depends: ocqt562+240-libqt5concurrent5 but it is not installable
Depends: ocqt562+240-libqt5core5a but it is not installable
Depends: ocqt562+240-libqt5dbus5 but it is not installable
Depends: ocqt562+240-libqt5gui5 but it is not installable
Depends: ocqt562+240-libqt5keychain1 but it is not installable
Depends: ocqt562+240-libqt5network5 but it is not installable
Depends: ocqt562+240-libqt5sql5 but it is not installable
Depends: ocqt562+240-libqt5webkit5 but it is not installable
Depends: ocqt562+240-libqt5widgets5 but it is not installable
Depends: ocqt562+240-libqt5xml5 but it is not installable
Depends: owncloud-client-l10n but it is not going to be installed
Depends: ocqt562+240-libqt5sql5-sqlite but it is not installable
These are not available in the debian repository.