sktest: 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/sbin/sktest - from package: libatasmart

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

On Debian possible default full paths are:

/usr/sbin/sktest - from package: libatasmart-bin

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

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