unregister-theme: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/share/gforge/bin/unregister-theme - from package: gforge-db-postgresql

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