nginx as the load balance use upstream_fair

[PstreeM China]

hello  expert:

need your help for this issue, from the google can not find any
similar case.
use freebsd 12.2 system. and install nginx by ports, the version
is:nginx-1.18.0_47,2

configuration the fair as the load balance policy, but there is some
mistake when i start the service like:
===============
root at env:/usr/local/etc/nginx # service nginx onerestart
Performing sanity check on nginx configuration:
nginx: [emerg] "fair" directive is not allowed here in
/usr/local/etc/nginx/nginx.conf:35
nginx: configuration file /usr/local/etc/nginx/nginx.conf test failed
===============

then,test if cancel the config "#fair", it running well.
===============
Performing sanity check on nginx configuration:
nginx: the configuration file /usr/local/etc/nginx/nginx.conf syntax is ok
nginx: configuration file /usr/local/etc/nginx/nginx.conf test is successful
Starting nginx.
 ==============

search from google and read the guide from nginx official site, the example
configure is all like this.  i don't know how to fix this ? thanks very
much for your help.

the part of configuration as below:
---------------------
load_module /usr/local/libexec/nginx/ngx_stream_module.so;
load_module /usr/local/libexec/nginx/ngx_http_dynamic_upstream_module.so;
load_module /usr/local/libexec/nginx/ngx_http_upstream_fair_module.so;
load_module /usr/local/libexec/nginx/ngx_http_sticky_module.so;

stream {

        upstream backend {
                server server1:443;
                server server2:443;
                server server3:443;
                fair;
        }

        server {
                listen       4443;
                proxy_pass      backend;

        }
}
-------------------------

BR//Armin