已解决，因为自己的粗心大意加熬夜修仙，公示自己犯的无脑错误如下：
- ubuntu 18.04.3 本身没问题，安装好干净系统后，没有把源设置为国内的（如 清华大学 https://mirrors.tuna.tsinghua.edu.cn/help/ubuntu/）导致更新速度慢的要死，冤枉熬夜（活该）。
- 下载seafile PRO server 服务器文件时，没有仔细看目录，明明是 [seafile-pro-server_7.0.9_x86-64.tar.gz]结果眼花看到下面的 [seafile-pro-server_7.0.9_x86-64_Ubuntu.tar.gz] 还怪人家写文档的没更新（安装的时候安装程序提示要前面那个文件，还自作聪明的把后面那个改名）（无语
） - 改正以上两个错误，现在在本地虚拟机顺利安装完成。
总结： 别人安装都没问题，就不要怀疑官方文档没更新，多怀疑自己！
最后，感谢帮助我的 hulk 朋友，谢谢！
===============已解决！=================
root@seafile_pro_servers:~# service seafile-server start
Job for seafile-server.service failed because the control process exited with error code.
See “systemctl status seafile-server.service” and “journalctl -xe” for details.
root@seafile_pro_servers:~# systemctl status seafile-server.service
● seafile-server.service - LSB: Seafile server
Loaded: loaded (/etc/init.d/seafile-server; generated)
Active: failed (Result: exit-code) since Thu 2019-10-10 18:16:32 UTC; 2min 16s ago
Docs: man:systemd-sysv-generator(8)
Process: 17473 ExecStart=/etc/init.d/seafile-server start (code=exited, status=1/FAILURE)
Tasks: 65 (limit: 4660)
CGroup: /system.slice/seafile-server.service
├─17287 /opt/seafile/seafile-pro-server-7.0.9/seafile/bin/seafile-controller -c /opt/seafile/ccnet -d /opt/seafile/seafile-data -F /opt/seafile/conf
├─17290 ccnet-server -F /opt/seafile/conf -c /opt/seafile/ccnet -f /opt/seafile/logs/ccnet.log -d -L /opt/seafile -P /opt/seafile/pids/ccnet.pid
├─17291 seaf-server -F /opt/seafile/conf -c /opt/seafile/ccnet -d /opt/seafile/seafile-data -l /opt/seafile/logs/seafile.log -P /opt/seafile/pids/seaf-server.pid -f -L
├─17292 /usr/bin/python2.7 -m seafevents.main --config-file /opt/seafile/conf/seafevents.conf --logfile /opt/seafile/logs/seafevents.log -P /opt/seafile/pids/seafevents
├─17294 /usr/bin/python2.7 -m wsgidav.server.run_server runfcgi --log-file /opt/seafile/logs/seafdav.log --pid /opt/seafile/pids/seafdav.pid --port 8080 --host localhos
└─17295 /usr/bin/java -Xms512m -Xmx1g -XX:-AssumeMP -XX:+UseConcMarkSweepGC -XX:CMSInitiatingOccupancyFraction=75 -XX:+UseCMSInitiatingOccupancyOnly -XX:+AlwaysPreTouch
Oct 10 18:16:31 seafile_pro_servers systemd[1]: This usually indicates unclean termination of a previous run, or service implementation deficiencies.
Oct 10 18:16:31 seafile_pro_servers systemd[1]: seafile-server.service: Found left-over process 17294 (python2.7) in control group while starting unit. Ignoring.
Oct 10 18:16:31 seafile_pro_servers systemd[1]: This usually indicates unclean termination of a previous run, or service implementation deficiencies.
Oct 10 18:16:31 seafile_pro_servers systemd[1]: seafile-server.service: Found left-over process 17295 (java) in control group while starting unit. Ignoring.
Oct 10 18:16:31 seafile_pro_servers systemd[1]: This usually indicates unclean termination of a previous run, or service implementation deficiencies.
Oct 10 18:16:31 seafile_pro_servers systemd[1]: Starting LSB: Seafile server…
Oct 10 18:16:32 seafile_pro_servers sudo[17482]: pam_unix(sudo:session): session closed for user seafile
Oct 10 18:16:32 seafile_pro_servers systemd[1]: seafile-server.service: Control process exited, code=exited status=1
Oct 10 18:16:32 seafile_pro_servers systemd[1]: seafile-server.service: Failed with result ‘exit-code’.
Oct 10 18:16:32 seafile_pro_servers systemd[1]: Failed to start LSB: Seafile server.
lines 1-24/24 (END)
