at: 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/at - from package: at

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

On Debian possible default full paths are:

/usr/bin/at - from package: at

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