radeontop: 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 Debian possible default full paths are:

/usr/sbin/radeontop - from package: radeontop

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

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