mii-diag: 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:

/sbin/mii-diag - from package: net-tools

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

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