memtester: 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/memtester - from package: memtester

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

On Debian possible default full paths are:

/usr/sbin/memtester - from package: memtester

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

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