fof-prover9: 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/fof-prover9 - from package: ladr4-apps

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

Error in other languages:
fof-prover9: bevel nie gevind nie
fof-prover9: comandă negăsită
fof-prover9: comando não encontrado
fof-prover9: commande introuvable
fof-prover9: command not found
fof-prover9: käsku ei ole
fof-prover9: không tìm thấy lệnh
fof-prover9: komanda nerasta
fof-prover9: Kommando nicht gefunden.
fof-prover9: kommandot finns inte
fof-prover9: komut yok
fof-prover9: nie znaleziono polecenia
fof-prover9: níor aimsíodh an t-ordú
fof-prover9: no se encontró la orden
fof-prover9: no s'ha trobat l'ordre
fof-prover9: opdracht niet gevonden
fof-prover9: parancs nem található
fof-prover9: perintah tidak ditemukan
fof-prover9: príkaz nenájdený
fof-prover9: příkaz nenalezen
fof-prover9: команда не найдена
fof-prover9: командата не е открита
fof-prover9: 命令找不到
fof-prover9: コマンドが見つかりません