mat: 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/mat - from package: mat

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

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