mtr: 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/sbin/mtr - from package: mtr

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

On Debian possible default full paths are:

/usr/bin/mtr - from package: mtr-tiny
/usr/bin/mtr - from package: mtr

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

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