udunits2: 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/udunits2 - from package: udunits2

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

On Debian possible default full paths are:

/usr/bin/udunits2 - from package: udunits-bin

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

Error in other languages:
udunits2: bevel nie gevind nie
udunits2: comandă negăsită
udunits2: comando não encontrado
udunits2: commande introuvable
udunits2: command not found
udunits2: käsku ei ole
udunits2: không tìm thấy lệnh
udunits2: komanda nerasta
udunits2: Kommando nicht gefunden.
udunits2: kommandot finns inte
udunits2: komut yok
udunits2: nie znaleziono polecenia
udunits2: níor aimsíodh an t-ordú
udunits2: no se encontró la orden
udunits2: no s'ha trobat l'ordre
udunits2: opdracht niet gevonden
udunits2: parancs nem található
udunits2: perintah tidak ditemukan
udunits2: príkaz nenájdený
udunits2: příkaz nenalezen
udunits2: команда не найдена
udunits2: командата не е открита
udunits2: 命令找不到
udunits2: コマンドが見つかりません