Create a Custom Trait - Sims 4 - Zerbu's Mod Constructor V4 Tutorial

I'm trying to use this application to make a Trait, but I keep getting the "Only lower priority components can be added to an element while it's in the middle of being built."

I'm looking for a comprehensive tutorial - initially I read there was a pdf file but I can't find it anywhere on the page where I downloaded it. Does anyone know why I'm getting this issue and what could fix it?

I'ts appearing when I try to add loots...