dfg2tptp: 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/dfg2tptp - from package: spass

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

Error in other languages:
dfg2tptp: bevel nie gevind nie
dfg2tptp: comandă negăsită
dfg2tptp: comando não encontrado
dfg2tptp: commande introuvable
dfg2tptp: command not found
dfg2tptp: käsku ei ole
dfg2tptp: không tìm thấy lệnh
dfg2tptp: komanda nerasta
dfg2tptp: Kommando nicht gefunden.
dfg2tptp: kommandot finns inte
dfg2tptp: komut yok
dfg2tptp: nie znaleziono polecenia
dfg2tptp: níor aimsíodh an t-ordú
dfg2tptp: no se encontró la orden
dfg2tptp: no s'ha trobat l'ordre
dfg2tptp: opdracht niet gevonden
dfg2tptp: parancs nem található
dfg2tptp: perintah tidak ditemukan
dfg2tptp: príkaz nenájdený
dfg2tptp: příkaz nenalezen
dfg2tptp: команда не найдена
dfg2tptp: командата не е открита
dfg2tptp: 命令找不到
dfg2tptp: コマンドが見つかりません