ttree: 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 CentOS possible default full paths are:

/usr/bin/ttree - from package: perl-Template-Toolkit

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/ttree - from package: libtemplate-perl

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

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