mtp11-trexist: 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/mtp11-trexist - from package: libmtp11-examples

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

Error in other languages:
mtp11-trexist: bevel nie gevind nie
mtp11-trexist: comandă negăsită
mtp11-trexist: comando não encontrado
mtp11-trexist: commande introuvable
mtp11-trexist: command not found
mtp11-trexist: käsku ei ole
mtp11-trexist: không tìm thấy lệnh
mtp11-trexist: komanda nerasta
mtp11-trexist: Kommando nicht gefunden.
mtp11-trexist: kommandot finns inte
mtp11-trexist: komut yok
mtp11-trexist: nie znaleziono polecenia
mtp11-trexist: níor aimsíodh an t-ordú
mtp11-trexist: no se encontró la orden
mtp11-trexist: no s'ha trobat l'ordre
mtp11-trexist: opdracht niet gevonden
mtp11-trexist: parancs nem található
mtp11-trexist: perintah tidak ditemukan
mtp11-trexist: príkaz nenájdený
mtp11-trexist: příkaz nenalezen
mtp11-trexist: команда не найдена
mtp11-trexist: командата не е открита
mtp11-trexist: 命令找不到
mtp11-trexist: コマンドが見つかりません