使用rails simple_form嵌套属性无法将符号转换为整数错误

I've seen many similar questions, and looked at the answers, but nothing seems to be helping me and I've been working on this for a while now. The error is 'Can't convert symbol into integer'. My goal is to be able to create multiple sub_groups for each race. I'm just starting with trying to create one for the time being. Here's the relevant code...I've seen many similar questions, and looked at