firewall-cmd: 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/firewall-cmd - from package: firewalld

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

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