theta: 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/theta - from package: dsdp

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