With out being able to look at a copy of the workbook or a screen shot my guess is that the issue is with Discrete vs Continuous format of your date. This is indicated in Tableau by blue for discrete and green for continuous. You can only do a scatter plot when a continuous field is on both the rows and columns shelves.So for example in the image below I brought in the order date as continuous by right clicking and dragging the field to the shelf ans selecting month continuous, SUM(Sales) to rows, sub-category to detail and Category to color. I then right click on the sheet and select add trend line.
Gotcha. I can't send a screenshot of my data but switching to continuous has worked. Thanks!