viewsu: 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/viewsu - from package: umview

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

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