iperf: 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/iperf - from package: iperf

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

On Debian possible default full paths are:

/usr/bin/iperf - from package: iperf

You may install the required package with command apt-get install {package}

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