run_perftest_loopback: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/run_perftest_loopback - from package: perftest

You may install the required package with command yum install {package}

Error in other languages:
run_perftest_loopback: bevel nie gevind nie
run_perftest_loopback: comandă negăsită
run_perftest_loopback: comando não encontrado
run_perftest_loopback: commande introuvable
run_perftest_loopback: command not found
run_perftest_loopback: käsku ei ole
run_perftest_loopback: không tìm thấy lệnh
run_perftest_loopback: komanda nerasta
run_perftest_loopback: Kommando nicht gefunden.
run_perftest_loopback: kommandot finns inte
run_perftest_loopback: komut yok
run_perftest_loopback: nie znaleziono polecenia
run_perftest_loopback: níor aimsíodh an t-ordú
run_perftest_loopback: no se encontró la orden
run_perftest_loopback: no s'ha trobat l'ordre
run_perftest_loopback: opdracht niet gevonden
run_perftest_loopback: parancs nem található
run_perftest_loopback: perintah tidak ditemukan
run_perftest_loopback: príkaz nenájdený
run_perftest_loopback: příkaz nenalezen
run_perftest_loopback: команда не найдена
run_perftest_loopback: командата не е открита
run_perftest_loopback: 命令找不到
run_perftest_loopback: コマンドが見つかりません