Pedro, it's not simple task to install and maintenance the superdesk, I don't see a lot of stuff which should be in this log and it's strange, couse it mixed with other stuff too, I need just *clean* output of this command I posted you before, If you can't do this, I can't help you. Sorry.
> Job for nginx.service failed because the control process exited with error code. See "systemctl status nginx.service" and "journalctl -xe" for details.
for clean Ubuntu 16.04 server edition, there is no apache installed, so if you have apache you should disable it or uninstall it how you did before. After this one command, I posted before will install nginx with superdesk for you.