nunit-gui: 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
/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/nunit-gui - from package: nunit-gui

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