fastboot: 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/fastboot - from package: android-tools

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

On Debian possible default full paths are:

/usr/bin/fastboot - from package: android-tools-fastboot

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

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