cmatrix: 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/cmatrix - from package: cmatrix

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

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