aireplay-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/sbin/aireplay-ng - from package: aircrack-ng

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

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