Why does creocoder/yii2-nested-sets throw an error "treeAttribute" is false?

Question

Yii

I added categories using creocoder/yii2-nested-sets , if you select a parent, then everything is added, if without a parent, then it gives this
Can not create more than one root when "treeAttribute" is false.
What's not tact?
I understand correctly that tree is needed so that there are several categories, and trees grow from them, and without it there will be only one main category, and the rest will be its children