ddrescue: 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/ddrescue - from package: ddrescue

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

On Debian possible default full paths are:

/usr/bin/ddrescue - from package: gddrescue

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

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