djvmcvt: 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/djvmcvt - from package: djvulibre

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

On Debian possible default full paths are:

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

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

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