I am not familiar with systemd, but that error indicates that your startup script is missing. On init system, those scripts are located in /etc/rc.d/init.d/. I have no clue what the equivalent systemd location would be.
pi@hassbian:/home/homeassistant/.homeassistant $ sudo systemctl status homeassitant.service
● homeassitant.service
Loaded: not-found (Reason: No such file or directory)
Active: failed (Result: exit-code) since Sat 2017-11-25 17:17:20 CET; 1h 0min ago
Main PID: 3707 (code=exited, status=203/EXEC)
Nov 25 17:17:20 hassbian systemd[1]: Started Home Assistant.
Nov 25 17:17:20 hassbian systemd[1]: homeassitant.service: main process exited, code=exited, status=203/EXEC
Nov 25 17:17:20 hassbian systemd[1]: Unit homeassitant.service entered failed state.
Nov 25 17:17:55 hassbian systemd[1]: [/etc/systemd/system/homeassitant.service:9] Executable path is not absolute, ignoring: source /svr/homeassistant/bin/activate
pi@hassbian:/home/homeassistant/.homeassistant $