fw_printenv: 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/fw_printenv - from package: u-boot-tools

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

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