i3lock: 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/i3lock - from package: i3lock

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

On Debian possible default full paths are:

/usr/bin/i3lock - from package: i3lock

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

Error in other languages:
i3lock: bevel nie gevind nie
i3lock: comandă negăsită
i3lock: comando não encontrado
i3lock: commande introuvable
i3lock: command not found
i3lock: käsku ei ole
i3lock: không tìm thấy lệnh
i3lock: komanda nerasta
i3lock: Kommando nicht gefunden.
i3lock: kommandot finns inte
i3lock: komut yok
i3lock: nie znaleziono polecenia
i3lock: níor aimsíodh an t-ordú
i3lock: no se encontró la orden
i3lock: no s'ha trobat l'ordre
i3lock: opdracht niet gevonden
i3lock: parancs nem található
i3lock: perintah tidak ditemukan
i3lock: príkaz nenájdený
i3lock: příkaz nenalezen
i3lock: команда не найдена
i3lock: командата не е открита
i3lock: 命令找不到
i3lock: コマンドが見つかりません