$ lsb_release -a No LSB modules are available. Distributor ID: Debian Description: Debian GNU/Linux testing (buster) Release: testing Codename: buster
I realise that currently you Debian 7 [wheezy] (end-of-life May 2018), Debian 8 [jessie] (end-of-life June 2020) and Debian 9 [stretch]; I’d appreciate it if you could look into also support buster.
Otherwise the problem is:
$ sudo apt-get install seafile-gui Reading package lists... Done Building dependency tree Reading state information... Done 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: seafile-gui : Depends: libevent-2.0-5 (>= 2.0.10-stable) but it is not installable
Support timelines are from https://wiki.debian.org/LTS