Why does this error occur (Ubuntu 18.04/nginx 1.14.0)?

nginx.conf:21
user www-data;

error.log
2019/01/24 19:07:07 [warn] 3526#3526: the "user" directive makes sense only if the master process runs with super-user privileges, ignored in /etc/nginx/nginx.conf:21

# ps -axu |grep nginx
root 3439 0.0 0.2 360156 9352 ? Ss 19:07 0:00 nginx: master process /usr/sbin/nginx -g daemon on; master_process on;
www-data 3441 0.1 0.5 364044 20716 ? S 19:07 0:00 nginx: worker process
www-data 3442 0.2 0.5 364560 21248 ? S 19:07 0:00 nginx: worker process
www-data 3443 0.0 0.3 362456 14852 ? S 19:07 0:00 nginx: cache manager process



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