numsum: 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 Debian possible default full paths are:

/usr/bin/numsum - from package: num-utils

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