swapon: 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/swapon - from package: util-linux-ng

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

On Debian possible default full paths are:

/sbin/swapon - from package: mount

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

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