sysctl: 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:

/sbin/sysctl - from package: procps

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

On Debian possible default full paths are:

/sbin/sysctl - from package: procps

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

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