units: 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/units - from package: units

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

On Debian possible default full paths are:

/usr/bin/units - from package: units

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

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