sendfax: 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/sendfax - from package: hylafax+-client
/usr/sbin/sendfax - from package: mgetty-sendfax

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

On Debian possible default full paths are:

/usr/bin/sendfax - from package: hylafax-client
/usr/sbin/sendfax - from package: mgetty-fax

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

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