Dates Table Relationship

What this message is expressing is that in the context of your entire data model, adding a relationship specifically between those 2 tables can't be done because the relationship already exists inside the data model, just more indirectly.

For example, if you had 3 tables: A, B and C.  If A -> B and B -> C is what you have right now.  You are trying to link A -> C, but because you can already get to the C through B, it isn't letting to link it again.

There are examples where you could do A -> B, A ->C and B -> C, but this depends on the type of relationships used.

I would urge you to review your entire data model, the flow of filters and the 1-1, 1-* type relationships.  You might that that you already have the relationship you need, or perhaps there is an issue in your model that might give you data integrity problems.