i3-save-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/i3-save-tree - from package: i3-wm

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

Error in other languages:
i3-save-tree: bevel nie gevind nie
i3-save-tree: comandă negăsită
i3-save-tree: comando não encontrado
i3-save-tree: commande introuvable
i3-save-tree: command not found
i3-save-tree: käsku ei ole
i3-save-tree: không tìm thấy lệnh
i3-save-tree: komanda nerasta
i3-save-tree: Kommando nicht gefunden.
i3-save-tree: kommandot finns inte
i3-save-tree: komut yok
i3-save-tree: nie znaleziono polecenia
i3-save-tree: níor aimsíodh an t-ordú
i3-save-tree: no se encontró la orden
i3-save-tree: no s'ha trobat l'ordre
i3-save-tree: opdracht niet gevonden
i3-save-tree: parancs nem található
i3-save-tree: perintah tidak ditemukan
i3-save-tree: príkaz nenájdený
i3-save-tree: příkaz nenalezen
i3-save-tree: команда не найдена
i3-save-tree: командата не е открита
i3-save-tree: 命令找不到
i3-save-tree: コマンドが見つかりません