couldn't start vnc server

I followed tutorial on this link to access GUI on droplet running Ubuntu 16.01

https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-vnc-on-ubuntu-16-04

But in Step 4 — Creating a VNC Service File when I try to start with

sudo systemctl start vncserver@1

I’m getting this error

Job for vncserver@1.service failed because the control process exited with error code. See "systemctl status vncserver@1.service" and "journalctl -xe" for details.

also I’m getting service status as below

vncserver@1.service - Start TightVNC server at startup
   Loaded: loaded (/etc/systemd/system/vncserver@.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Sun 2018-03-25 12:35:37 UTC; 12s ago
  Process: 26404 ExecStart=/usr/bin/vncserver -depth 24 -geometry 1280x800 :%i (code=exited, status=217/USER)
  Process: 26401 ExecStartPre=/usr/bin/vncserver -kill :%i > /dev/null 2>&1 (code=exited, status=217/USER)

Mar 25 12:35:37 waspy systemd[1]: Starting Start TightVNC server at startup...
Mar 25 12:35:37 waspy systemd[26401]: vncserver@1.service: Failed at step USER spawning /usr/bin/vncserver: No such process
Mar 25 12:35:37 waspy systemd[26404]: vncserver@1.service: Failed at step USER spawning /usr/bin/vncserver: No such process
Mar 25 12:35:37 waspy systemd[1]: vncserver@1.service: Control process exited, code=exited status=217
Mar 25 12:35:37 waspy systemd[1]: Failed to start Start TightVNC server at startup.
Mar 25 12:35:37 waspy systemd[1]: vncserver@1.service: Unit entered failed state.
Mar 25 12:35:37 waspy systemd[1]: vncserver@1.service: Failed with result 'exit-code'.