A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data
In Excel Online, it's true that you might not find the exact "Group Data" option as it is present in the desktop version of Excel. Excel Online, being a web-based version, tends to have a more streamlined interface with fewer advanced features compared to the desktop application.
You can group date from Excel Desktop and use it on Excel Online.
If you haven't desktop application, you can add a helper column and use formula to get month value & year value. Then you can use it in your povit table.
Grouping data is highly useful for data analysis and reporting. Hope Microsoft will add this feature on Excel online in the near future.