Marco Russo and Alberto Ferrari explain an error:
When you create a Power BI matrix, you drag and drop columns in the matrix, then add some measures, and Power BI figures out on its own which combinations of values to show. The process is so intuitive that we mostly ignore the details. However, Power BI sometimes cannot figure out how to populate the matrix, thus producing the error: “can’t determine relationship between the fields”. Adding a measure fixes the problem, but why? In some other scenarios, Power BI shows many empty rows, eliminating many of them only when you add a measure. Power BI shows a subset of the values in other scenarios, even when no measure is involved.
Read on for the explanation.