dnstxt: 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 CentOS possible default full paths are:

/usr/bin/dnstxt - from package: ndjbdns

You may install the required package with command yum install {package}

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