gnome-tweak-tool: 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/gnome-tweak-tool - from package: gnome-tweak-tool

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

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