fabio_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/fabio_viewer - from package: python-fabio

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

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