flock: 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/flock - from package: util-linux-ng

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

On Debian possible default full paths are:

/usr/bin/flock - from package: util-linux

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

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