sync: 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:

/bin/sync - from package: coreutils

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

On Debian possible default full paths are:

/bin/sync - from package: coreutils
/usr/lib/klibc/bin/sync - from package: klibc-utils

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

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