i3-nagbar: 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 Debian possible default full paths are:

/usr/bin/i3-nagbar - from package: i3-wm

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

Error in other languages:
i3-nagbar: bevel nie gevind nie
i3-nagbar: comandă negăsită
i3-nagbar: comando não encontrado
i3-nagbar: commande introuvable
i3-nagbar: command not found
i3-nagbar: käsku ei ole
i3-nagbar: không tìm thấy lệnh
i3-nagbar: komanda nerasta
i3-nagbar: Kommando nicht gefunden.
i3-nagbar: kommandot finns inte
i3-nagbar: komut yok
i3-nagbar: nie znaleziono polecenia
i3-nagbar: níor aimsíodh an t-ordú
i3-nagbar: no se encontró la orden
i3-nagbar: no s'ha trobat l'ordre
i3-nagbar: opdracht niet gevonden
i3-nagbar: parancs nem található
i3-nagbar: perintah tidak ditemukan
i3-nagbar: príkaz nenájdený
i3-nagbar: příkaz nenalezen
i3-nagbar: команда не найдена
i3-nagbar: командата не е открита
i3-nagbar: 命令找不到
i3-nagbar: コマンドが見つかりません