% Difference from the previous quarter

This is actually difficult to help you with such limited information.   

What is the meaning of linking target to report?   I will need to understand the business objective of what you are trying to do here.     

The issue here and the error you are getting  is that it is unable to return a single value for your measure which means your relationships between your tables are returning multiple rows rather than just one value, which in the case of a measure, it requires one value.

This problem almost always lies in how the data is modelled.

You will need to explain what you are actually trying to do, i think more business context is required here.

I personally would never link it that way;

calendar should be connect to report separately

calendar should be connected to target separately  

but without understanding  what you are trying to measure, and why target sits in the middle the technical only information only you have provided isn't enough to help you solve this problem.

hope that makes sense.