diagram-viewer: 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/diagram-viewer - from package: tklib

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

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