Edit Change Item - Type Add greyed out - Why?

I am completing the course “Expand Your Domain Modeling Skills” and I am currently at “3.3.2 Build a Scoring System”.    The yellow marked thing is what I am trying to achieve (“Add”).   And this is from my own project, what i currently have:     And as you can see, the Add Option is greyed out. Why?   Thank you in advance.
1 answers

you need to change the type of your association (in the domain model)

see here : https://docs.mendix.com/refguide/associations/

What you probably need is a many to many association