ddjvu: 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/ddjvu - from package: djvulibre

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

On Debian possible default full paths are:

/usr/bin/ddjvu - from package: djvulibre-bin

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

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