The problem is that the trendline feature only works with continuous data, and something like “Jan 2014"” is categorical, not continuous. So, I think you have 3 options:
You can create a numeric YearMonth column in your Date table, e.g., 201401, 201402, etc. and use that as your x-axis column. Easy fix, but doesn’t look great on your visual; or
Instead of the auto trendline feature, you can develop a measure that will estimate a linear regression line for your data and then add that to your line chart as a third data series. This will allow you to use a categorical x axis.
Create the visual via an R script and import it into Power BI.
If you choose option #2, this video may be quite useful:
I hope this is helpful.