sing: 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/sing - from package: sing

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

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