isnum: 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/bin/isnum - from package: wcstools

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

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