mail-unlock: 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/mail-unlock - from package: lockfile-progs

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

On Debian possible default full paths are:

/usr/bin/mail-unlock - from package: lockfile-progs

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

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