grass64: 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/bin/grass64 - from package: grass

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

On Debian possible default full paths are:

/usr/bin/grass64 - from package: grass-core

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

Error in other languages:
grass64: bevel nie gevind nie
grass64: comandă negăsită
grass64: comando não encontrado
grass64: commande introuvable
grass64: command not found
grass64: käsku ei ole
grass64: không tìm thấy lệnh
grass64: komanda nerasta
grass64: Kommando nicht gefunden.
grass64: kommandot finns inte
grass64: komut yok
grass64: nie znaleziono polecenia
grass64: níor aimsíodh an t-ordú
grass64: no se encontró la orden
grass64: no s'ha trobat l'ordre
grass64: opdracht niet gevonden
grass64: parancs nem található
grass64: perintah tidak ditemukan
grass64: príkaz nenájdený
grass64: příkaz nenalezen
grass64: команда не найдена
grass64: командата не е открита
grass64: 命令找不到
grass64: コマンドが見つかりません