kestrel-hellomvc.service: Failed at step USER spawning /usr/bin/dotnet: No such process

In file /etc/systemd/system/gunicorn.service, I had added user. The issue got resolved when I removed the user.

...

[Service]
User="someuser"
...

Source: https://superuser.com/questions/1156676/what-causes-systemd-failed-at-step-user-spawning-usr-sbin-opendkim-no-such-p

I've solved the problem, the source folder owner should be www-data, having 755 permission. The Apache user is called www-data on Ubuntu, and apache on CentOS. So specifically set User=www-data in kestrel-hellomvc.service if running on Ubuntu, thanks to Jeppe's comment.