ucalc: 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/ucalc - from package: perl-Math-Calc-Units

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

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