matho-sumsq: 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/matho-sumsq - from package: mathomatic-primes

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

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