nail: 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/nail - from package: mailx

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

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