fax: 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 CentOS possible default full paths are:

/usr/bin/fax - from package: efax

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

On Debian possible default full paths are:

/usr/bin/fax - from package: efax

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