CDM Icon for creating association disabled

Hi All,

I'm new to PD.  I am trying to create new Conceptual Data Model. I have created a couple of entities and I want to bind them together with an association, but the icon for creating an association is disabled and also don't see the association under "Model" menu option. Together with association Inheritance and Association Link are disabled.

Would you know what might be the reason for that? If I open an example CDM from SAP I can use all the objects. Do I have to change sth in model properties?

Thanks and Best regards,

Kasia