atd: 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/sbin/atd - from package: at

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

On Debian possible default full paths are:

/usr/sbin/atd - from package: at

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

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