Order of Operations Power BI visualization

Hello, can someone tell me the order that the data types should be pulled into a visual? Should I pull the data files in first and then the reference files? I put a table into the report and pulled in reference column then pulled in the data point column, but i got an error. When I pulled in the data point first then the reference data point, it worked fine. 

My reference file contains customer number, data contains customer number so I created that relationship.