Tableau's Trend Lines functionality relies on doing a statistical regression analysis, which is dependent on two continuous fields (dates, numbers). With categories we can't perform this type of ordinal calculation in order to generate the regression trendline.
That said, looking at your example, it looks like we might simply want to showcase values as a line chart on top of our bars to represent to the direction of our values (a general "trend" and not regression analysis). You can do this with a Combination Charts. I've attached your original workbook with this example included.
Trend Line Question .twbx 9.6 KB