doublecmd: 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 Debian possible default full paths are:

/usr/bin/doublecmd - from package: doublecmd-common

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

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