When I googled it, Tableau has a page that says to update but I have Tableau 2018.2. Is it because I'm joining Excel and CSV? All of the fields being called in the workbook/LOD are from one workbook. Any ideas?
Can you show a screenshot of the data source screen? Or provide the workbook? I've joined Excel and CSV before, and not had trouble. Are you doing anything unusual in the join, like a join calculation?