spanning_tree: 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/spanning_tree - from package: vowpal-wabbit

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

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