udunits: command not found

Have You tried accessing the binary with the full path?

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/udunits - from package: udunits

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