radwho: 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/radwho - from package: freeradius-utils

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

On Debian possible default full paths are:

/usr/bin/radwho - from package: freeradius-utils

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