Two active relationships between different tables (UniqueID and Dates)

Hi JOHNPINTOC 

The reason is simple, it's because of bidirectional relationships.
Active relationship: Headerid <> Monthlyprod 

What would happen if you could create a relationship between Dates and Monthlyprod:
Headerid  <> Portafoli_m <> Dates  <> Monthlyprod 

Monthlyprod is currently filtered by Headerid and Headerid filters Portafoli_m that filters Date so you can not create a relationship with Dates because it would mean Headerid has  two active relationships with Monthlyprod 

Solutions:

You can change the relationship between Portafoli_m and  Dates to the single direction where Dates filters Portafoli_m.

Or adhere to best practice and avoid bidirectional relationships altogether.