fdnstxtlist: 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/fdnstxtlist - from package: firedns

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

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