A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Hello,
some aspects of Excel can only be created in Excel for desktop. That includes adding a trendline to a chart. This cannot be done in Excel online. Edit the Excel workbook in Excel and add the trendline there. After that, you can open the workbook in Excel online and can see the trendline, but you can't create the trendline in Excel online.