First install NRPE on the Nagios server and on the client to monitor.

On the client, if you have multiple ethernet adapter specify on which the NRPE demon will LISTEN (server_address=192.168.222.152)

Enable the log in verbose mode to be able to debug, once everything is right that can be changed.
Parameters: log_facility=daemon and debug=1

On the client start the nrpe demon

/etc/rc.d/init.d/nrpe start

On the Nagios server try to contact the client:

[root@nagios objects]# /usr/lib/nagios/plugins/check_nrpe  -H 192.168.222.152
CHECK_NRPE: Error - Could not complete SSL handshake.

You have to authorize the Nagios server to contact the client. On the client in /etc/nagios/nrpe.cfg

allowed_hosts=127.0.0.1,192.168.222.168

Restart you nrpe demon on the client:

/etc/rc.d/init.d/nrpe restart

On the Nagios server try to contact the client:

[root@nagios objects]# /usr/lib/nagios/plugins/check_nrpe  -H 192.168.222.152
NRPE v2.15

Now it work !

Try another test:

[root@nagios objects]# /usr/lib/nagios/plugins/check_nrpe  -H 192.168.222.152 -c check_load
OK - load average: 1.30, 1.35, 1.44|load1=1.300;15.000;30.000;0; load5=1.350;10.000;25.000;0; load15=1.440;5.000;20.000;0; 
[root@nagios objects]# /usr/lib/nagios/plugins/check_nrpe  -H 192.168.222.152 -c check_total_procs
PROCS OK: 62 processes | procs=62;150;200;0;

Threshold and command are defined on the client into /etc/nagios/nrpe.cfg

...
command[check_users]=/opt/freeware/lib/nagios/plugins/check_users -w 5 -c 10
command[check_load]=/opt/freeware/lib/nagios/plugins/check_load -w 15,10,5 -c 30,25,20
command[check_hda1]=/opt/freeware/lib/nagios/plugins/check_disk -w 20% -c 10% -p /dev/hda1
command[check_zombie_procs]=/opt/freeware/lib/nagios/plugins/check_procs -w 5 -c 10 -s Z
command[check_total_procs]=/opt/freeware/lib/nagios/plugins/check_procs -w 150 -c 200 

Now you can add on the Nagios server into command.cfg

# 'check_nrpe_load' command definition
define command{
      command_name    check_nrpe_load
      command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -c check_load
      }

# 'check_nrpe_procs' command definition
define command{
      command_name    check_nrpe_procs
      command_line    $USER1$/check_nrpe -H $HOSTADDRESS$ -c check_total_procs
      }

And in services.cfg

define service{
      use                             aix-service
      hostgroup_name                  aix
      service_description             CPU Load
      check_command                   check_nrpe_load
      }

define service{
      use                             aix-service
      hostgroup_name                  aix
      service_description             Total Processes
      check_command                   check_nrpe_procs
      }

# nagios -v /etc/nagios/nagios.cfg
# /etc/rc.d/init.d/nagios reload

On AIX binary files or RPM start NRPE as root

To change it, recompile nrpe.c

Get latest source from github, and modify using the followin paragraph, starting line 2587 on version 4.1.0

#endif
/* Last Modified: 2022-07-18 line 2587 */

                        if (full_drop) {
                                /*setuid(uid) doesn’t work on aix but doesn’t return -1 either*/
                                if (setuid(uid) == -1) 
                                        logit(LOG_ERR, "Warning: Could not set UID=%d", (int)uid);
                                /*setuid(uid) returned anything but -1*/
                                else 
                                        /*if the current uid still isn’t geteuid() setuid failed and thus we SETEUID(uid)*/
                                        if (uid != geteuid()) 
                                                if (SETEUID(uid) == -1)
                                                        logit(LOG_ERR, "Warning: Could not set effective UID=%d", (int)uid);

                        } else if (SETEUID(uid) == -1)
                                logit(LOG_ERR, "Warning: Could not set effective UID=%d", (int)uid);
                }