atop: 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/atop - from package: atop

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

On Debian possible default full paths are:

/usr/bin/atop - from package: atop

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

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