i.fft: 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 CentOS possible default full paths are:

/usr/lib64/grass-6.4.4/bin/i.fft - from package: grass

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/lib/grass64/bin/i.fft - from package: grass-core

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

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