Thanks for editing your post. But really, I meant the code and error messages would be in separate code tags, one for the code and one for the error messages.
The bad news is I know nothing about the mkr series Arduino. I always looked at them and thought "who wants that?" because they seem over complicated and expensive.
For your project I would have chosen an esp8266 board. Wemos Mini are my favourite. But then I know little about dmx either, so you had better get a second opinion. This is the Arduino forum, so I'm sure one will be along in a moment...