showtree: 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/showtree - from package: prime-phylo

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

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