zerofree: 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/sbin/zerofree - from package: zerofree

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

On Debian possible default full paths are:

/usr/sbin/zerofree - from package: zerofree

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

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