airdecap-ng: 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/airdecap-ng - from package: aircrack-ng

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

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