fdnstxt: 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/fdnstxt - from package: firedns

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

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