viewfax: 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 CentOS possible default full paths are:

/usr/bin/viewfax - from package: mgetty-viewfax

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/viewfax - from package: mgetty-viewfax

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

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