rtl8139-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 Debian possible default full paths are:

/usr/sbin/rtl8139-diag - from package: nictools-pci

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

Error in other languages:
rtl8139-diag: bevel nie gevind nie
rtl8139-diag: comandă negăsită
rtl8139-diag: comando não encontrado
rtl8139-diag: commande introuvable
rtl8139-diag: command not found
rtl8139-diag: käsku ei ole
rtl8139-diag: không tìm thấy lệnh
rtl8139-diag: komanda nerasta
rtl8139-diag: Kommando nicht gefunden.
rtl8139-diag: kommandot finns inte
rtl8139-diag: komut yok
rtl8139-diag: nie znaleziono polecenia
rtl8139-diag: níor aimsíodh an t-ordú
rtl8139-diag: no se encontró la orden
rtl8139-diag: no s'ha trobat l'ordre
rtl8139-diag: opdracht niet gevonden
rtl8139-diag: parancs nem található
rtl8139-diag: perintah tidak ditemukan
rtl8139-diag: príkaz nenájdený
rtl8139-diag: příkaz nenalezen
rtl8139-diag: команда не найдена
rtl8139-diag: командата не е открита
rtl8139-diag: 命令找不到
rtl8139-diag: コマンドが見つかりません