besside-ng: command not found

Have You tried accessing the binary with the full path?

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/besside-ng - from package: aircrack-ng

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