One time you have completed the installation Nagios-plugin and nrpe , if you find an error on the Nagios monitoring page when configuring in the client system,

Current Status:         UNKNOWN  (for 0d 0h 3m 13s)
Status Information: /bin/ping -n -U -w 30 -c 5
CRITICAL - Could not interpret output from ping comman

We can easily fixed this issue just execute below commands,

Check file permission:
# ls -al /bin/ping
  -rwxr-xr-x 1 root root 35864 Dec 21  2006 /bin/ping*

Give permission to that executable file,
# chmod u+s /bin/ping

u => File owner.
s => Set-user-ID-on-execution permission if the u flag is specified
# ls -al /bin/ping
-rwsr-xr-x 1 root root 35864 Dec 21  2006 /bin/ping*

Following command returns the output "ping OK"
# /usr/local/nagios/libexec/check_ping -H  -w 3000.0,80% -c 5000.0,100% -p 5

PING OK - Packet loss = 0%, RTA = 0.09 ms|rta=0.092000ms;3000.000000;5000.000000;0.000000 pl=0%;80;100;0


Current Status:         OK (for 0d 0h 56m 13s)
Status Information:  PING OK - Packet loss = 0%, RTA = 0.07 ms
Performance Data:    rta=0.067000ms;3000.000000;5000.000000;0.000000 pl=0%;80;100;0

Nagios Server Side configuration for that particular host,
# vim /usr/local/nagios/etc/objects/host4.hostname.cfg

define service{
            use                                 generic-service
            service_description          Host Status
            check_command             check_ping! -H

Note : is your Client server ip address.